Gain switched audio conferencing network

ABSTRACT

An audio conferencing system includes a plurality of remote stations (10) which are interconnected in a conference mode through a central switching network (66) through communication lines (16). Each of the remote stations has a microphone (34) and a speaker (64) associated therewith. The microphone (34) on each of the remote stations has a voltage controlled amplifier (VCA) (59) associated therewith and the speaker (64) has a VCA (60) associated therewith. A CPU (48) selectively controls the output of the microphone (34) to selectively decrease the gain thereof when voice is not detected. When voice is detected above a predetermined threshold, the microphone (34) is allowed to operate at full gain and the speaker (64) has the output level thereof decreased. This effectively reduces the audio loop gain between remote stations in a conference.

TECHNICAL FIELD OF THE INVENTION

The present invention pertains in general to communication networks and,more particularly, to communication networks for interfacing betweenremote video terminals to provide video, audio and data pathstherebetween.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to patent application Ser. No. 721,281,filed Apr. 8, 1985, and patent application Ser. No. 721,083, filed Apr.8, 1985.

BACKGROUND OF THE INVENTION

Video communication has evolved over the years from a simple videotelephone concept to a sophisticated network for allowing multipleparties to enter into a video conference. A number of factors haveprevented total success of such prior video systems, including publicacceptance, excessive cost, system complexity and inadequate videoquality. Although these factors can be manipulated somewhat to providean improved system for video communications, inherent constraints suchas standardized video formats and presently existing communicationssystems minimize the design flexibility which may be utilized inachieving a feasible system.

Video communications can be utilized for a number of applications. Insome applications, only transmission of a hard copy is required, such asa picture or graphics representation. Such hard copy transmission isnormally accomplished by such techniques as facsimile transmission toenable transmission of such things as x-rays, flow charts, and the like.However, for a full video conferencing system wherein individuals desireboth an audible communication path and a real time visual communicationpath, it is necessary to provide full motion color video. Thetransmission of full motion color video normally requires a much greaterbandwidth than the transmission of facsimile data. This is due to thefact that the video is transmitted in "real time" to allow aninteractive conference.

Non real time video systems have been previously developed forteleconferencing which operate on lower bandwidth communication links.One type of non real time system is often referred to as the slow scanvideo system which is programmed to send new pictures at regularintervals, whether the image is changed or not. In this type of system,movement is prohibited in order to provide a relatively clear picture.An alternate to the slow scan system is the freeze-frame system whichrecords a clear picture of the speaker and transmits this clear pictureto the remote terminals in the conference at regular intervals. However,the slow scan and freeze-frame systems are not real time and the partiesviewing the conference are only provided a sequence of stepped poses forthe speaker.

Full motion video, heretofore the most preferred for video conferencing,is accomplished by a number of techniques. The most effective fullmotion video conferencing systems transmit over 1.544-megabits/s T1telephone lines. Although the picture quality on these high bandwidthsystems is high, so are the operational costs per hour for nationwidepoint-to-point connections.

To decrease the cost, data compressed video conferencing systems havebeen developed which operate on a 56-kb/s packet-switched network. Datacompression is required to operate on these 56-kb/s systems, sincedirect digitization of standard NTSC broadcast video color signalsrequires approximately 80 megabits per second, far beyond the capacityof most transmission lines. To transmit full-motion color at lowerbandwidths, the digital signal must be compressed by the removal ofredundant information.

Two main data compression approaches have been heretofore developed,namely interframe coding and intraframe coding. In interframe coding,successive video frames are compared, pixel by pixel, and only changedvalues are transmitted. In intraframe coding, values for entire blocksof pixels within a frame are transmitted as mathematical transforms.These techniques are useful for transmitting at 1.544-Mb/s over T1lines. However, transmission at 56-kb/s requires further datacompression. This is accomplished by squeezing out data on luminescence,hue, resolution and scan rate. A cosine transform is utilized tocompress the data efficiently, with the negative result being breakup ofthe picture into blocks of pixels when the transform needs time for eachrecalculation and the system has too many bits to send. Other prior56-kb/s systems use a binary algorithm that degrades by losingresolution when overwhelmed by too much motion.

The disadvantages to the 56-kb/s systems and similar digital systems arethat they first require the availability of a digital network and,secondly, they require a relatively expensive codec to interface betweenthe video terminal and the network. These systems are seldom availableor financially feasible for local or intrafacility use.

In prior video conferencing systems, another major disadvantage has beenthe initial cost of the remote terminals which are, at present,dedicated to video conferences. The longer a video terminal remainsidle, the higher the cost-per-hour. Therefore, it would be desirous tointegrate the remote video terminal with other functions to lower theeffective cost per hour of the video conferencing feature.

In view of the above disadvantages of prior video conferencing systems,there exists a need for a full motion, color video conferencing networkthat overcomes the deficiencies of the present systems and is moreadapted to economical local and intrafacility use.

SUMMARY OF THE INVENTION

The present invention disclosed and claimed herein comprises a methodand apparatus for communicating between remote stations. Each of theremote stations includes a microphone for receiving information from atransmission medium and a speaker for outputting the audio informationto the associated transmission medium. The associated transmissionmedium provides a feedback path between the microphone and the speaker.A communication link provides a first audio path between the microphoneand one of the remote stations and the speaker means at selected ones ofthe remaining remote stations. The communication link also provides asecond audio path between the microphones at the other selected remotestations and the speaker means at the one remote station, the first andsecond audio paths being isolated. Loss is selectively inserted into theaudio path at each of the remote stations such that the audio level ofthe received audio in the communication link that is transmitted fromone speaker to the associated one of the microphones and transmitted tothe communication link is decreased to prevent oscillation. Theinsertion of the loss adjusts as a function of the input signal level tothe microphone from the transmission medium.

In yet another embodiment of the present invention, each of the remotestations is connected to a centralized switching network for receivingthe audio information from each of the remote stations in theconference. This centralized station sums audio from all of the remotestations for transmission to a select one of the remote stations withthe exception of the audio from the selected remote stations. Each ofthe remote stations receives summed audio from all of the other remotestations in the conference. The amount of loss inserted into the audioloop is dependent on the number of remote stations in the conference,the loss being increased as more remote stations are added to theconference.

In yet another embodiment of the present invention, a variable gaindevice is disposed between both the microphone and the communicationlink and the speaker in the communication link. The microphoneattenuator is controlled in a first mode to insert a predeterminedamount of loss into the audio loop and the speaker attenuator iscontrolled to insert a second predetermined loss into the audio loop. Anexternal control is provided for allowing the user to set a desiredoutput level from the speaker. A voice detect circuit generates a voicedetect signal in response to the input level to the microphone exceedinga predetermined threshold. When the voice detect signal is generated,the associated remote station operates in the second mode to insert thesecond predetermined loss and decrease the output level of the speaker.When the voice detect signal is not present, the system operates in thefirst mode and decreases the transmitted output from the microphone andallows the speaker to output audio signals at the desired level.

In yet another embodiment of the present invention, a level threshold isprovided for the speaker output. When the selected level or the outputof the speaker is set below this level threshold, the remote station isinhibited from inserting either the first or the second predeterminedlosses.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying Drawings in which:

FIG. 1 illustrates a diagrammatic view of the Mate remote video terminaland work station combination;

FIG. 2 illustrates a schematic block diagram of the Mate remote videoterminal;

FIG. 3 illustrates a cluster of Mate video terminals connected togetherin the basis switching network;

FIG. 4a illustrates a schematic block diagram of the Mix switchingnetwork;

FIG. 4b illustrates the communication paths for the audio/video/datavideo conferencing information and parallel baseband information;

FIG. 5 illustrates a diagram of the transmission spectrum for the singlecable multiplexing format utilized for communication between the Mixswitching network and the Mate video terminals;

FIG. 6 illustrates a schematic block diagram of the single cablemultiplexer;

FIG. 7 illustrates a network of Mate video terminal clusters;

FIG. 8 illustrates a video conferencing network utilizing a basebandnetwork for interfacing data to the video terminals with a broadbandnetwork for interfacing audio and video therebetween;

FIG. 9 illustrates a broadband network for interfacing video, audio anddata thereover with a centralized controller provided for datainterface;

FIG. 10 illustrates a satellite link between networks;

FIG. 11 illustrates a block diagram of the interface between the Matevideo terminal and the broadband network;

FIG. 12 illustrates a block diagram of the audio video processor portionof the Mate;

FIG. 13 illustrates a diagrammatic view of the audio path with gainswitching;

FIG. 14 illustrates a diagrammatic view of the audio patch for amultiway conference with gain switching;

FIGS. 15a and 15b illustrate graphs of the switched gain for the audiopaths;

FIG. 16 illustrates a block diagram for the Mix switching networkprocessor;

FIG. 17 illustrates a block diagram of the interface board for theremote video terminal;

FIG. 18 illustrates a schematic of the interface network for interfacingthe video terminal with a local processor;

FIGS. 19A-F illustrate a schematic for the audio video processor portionof the Mate;

FIGS. 20a-20c illustrate schematics for the Mix switching networkprocessor;

FIG. 21 illustrates a flowchart for placing and receiving a call at theMate remote video terminal;

FIG. 22 illustrates a flowchart for adding a party to a video conferenceat the Mate remote video terminal;

FIG. 23 illustrates a flowchart for locking a conference onto the Materemote video terminal;

FIG. 24 illustrates a flowchart for generating the voice detect signal;

FIG. 25 illustrates a schematic block diagram of task scheduling in theMix switching network;

FIG. 26 illustrates a diagrammatic view of the video conferencingnetwork showing a switch plane and a data plane;

FIG. 27 illustrates a side view of the diagram of FIG. 26 with analternate configuration for the Network Master;

FIG. 28 illustrates a diagrammatic view of the data plane for a threemix network;

FIG. 29 illustrates a flowchart for the master task in the network forplacing a call;

FIG. 30 illustrates a flowchart for the master task when adding a partyto the conference;

FIG. 31 illustrates a flowchart for determining the priority of partiesin a conference in the master task of the network;

FIG. 32 illustrates a flowchart for the keep alive function between themaster and slave tasks in the switching networks; and

FIG. 33 illustrates a flowchart for communication between the master andslave tasks in the switching network.

DETAILED DESCRIPTION OF THE INVENTION Multifunction Work Station

Referring now to FIG. 1, there is illustrated a diagrammatic view of amultifunction work station. The work station includes a video terminal10 which includes a monitor display screen 12 and a viewfinder orsecondary display screen 14 for displaying video. The video terminal 10is hereinafter referred to as "Multimedia Terminal Equipment" or "Mate".The Mate 10 is utilized for processing audio and video information andalso data. The audio/video/data can be output to a video communicationnetwork through a single cable 16 to interface with other Mates atremote points. The audio/video/data is multiplexed onto the cable 16which is hereinafter referred to as the "SCM cable". In this mode, theMate 10 can comprise a portion of a video communication network whichcan be connected in a video conferencing mode, as will be describedhereinbelow.

The Mate 10 interfaces with the user through a station set interface box(SSIB) 18 through a serial data link 20. The SSIB 18 includes anintegral key pad for allowing the user to input data thereto in the formof key strokes. In addition, a standard telephone 22 with its associatedkey pad can be interfaced to the Mate 10 through an interface box 24. Inthe preferred embodiment, the interface box 24 may be integrated withthe SSIB 18 such that the telephone 22 may plug into the SSIB 18.Hereinafter, the interface box 24 will be considered a part of the SSIB18.

The Mate 10 includes internal thereto a video camera 32 for generating alocal video signal. In addition, an external camera 26 can be interfacedto the Mate 10 as well as other auxiliary audio/video devices such as avideo recorder (VCR) 28. As will be described hereinbelow, the videofrom either the internal video camera or the auxiliary devices 26 and 28can be utilized to either display video on the monitor 12, theviewfinder 14 or generate a video signal for transmission on the SCMcable 16.

The internal camera 32 shoots color television images using a fixedfocus wide angle lens. A microphone 34 is mounted on the front panel ofthe Mate 10 to allow audible interaction between the user and the Mate10. The video displayed on the monitor 12 is controlled by a mode switch36, the operation of which will be described hereinbelow. Indicators 38are provided to determine when video is being transmitted over the SCMcable 16 when power is applied and also when a privacy mode is selected.The privacy mode is enabled through a privacy switch 40. A mode switch41 is also provided that enables the user to select various functions ofthe Mate 10 in a sequential manner, as will be described hereinbelow.

The Mate 10 is operable to interface with a local processor 42 throughan interface circuit 44. A keyboard 46 is provided which is interfacedto the local processor 42. In the preferred embodiment, the type oflocal processor 42 utilized requires the keyboard 46 to interfacethrough a separate monitor. Since the Mate 10 serves as the interfacemonitor, the interface circuit 44 provides this interface in addition tointerfacing the video and data output of the local processor 42 with theMate 10. When the local processor 42 is utilized, the Mate 10 canoperate in the Slave mode and act as the monitor for the local processor42. In the Slave mode, the local processor 42 controls the majority ofthe functions of the Mate 10 wherein the Mate 10 operates as a monitorwith the additional capability of maintaining contact with the networkthrough the SCM cable 16. The interface circuit 44 is dependent upon thetype of local processor 42 utilized and is designed for each specificlocal processor.

The local processor 42 interfaces its video and data through a line 45to the interface box 44. The video is normally in the form of threesignals constituting the red, green and blue signals that drive themonitor and also the vertical and horizontal sync signals. The data isnormally comprised of some form of standardized serial link such as anRS-232 format. This data link can contain both control data and alsoaudio data in the form of synthesized sounds etc.

Local processors or other peripheral equipment which are utilized inconjunction with the Mate 10 to form the overall workstation normallyinclude the capability to interface with a baseband network such as alocal area network (LAN). The network of the present inventionaccommodates transmission of baseband information which is dedicated tooperation of the local processor 42 in a separate and distinct networkfrom that for video conferencing by allowing transmission of basebandinformation over the SCM cable 16. The local processor 42 is interfacedwith the interface box 44 through a baseband transmission line 47 whichis internally coupled to the SCM cable 16. The audio/video/data thatcorresponds to the video conferencing network is separate and distinctfrom the baseband information that is transmitted and received by thelocal processor 42. In this manner, only a single cable is required toaccommodate both a video conferencing network and also maintain thebaseband communication link, which is inherent to most localizedfacilities. In this manner, the video teleconferencing feature can beadded to an already existing LAN system with the operation of bothsystems remaining separate and distinct.

Referring now to FIG. 2, there is illustrated a schematic block diagramof the Mate 10. All of the data processing in the Mate 10 is performedby a Central Processing Unit (CPU) 48. The CPU 48 carries out video andaudio switching and controls audio levels in the Mate 10. The processorhas three 9600 baud serial ports that direct the actions of the Mate 10such that the functions thereof can be directly or indirectly controlledby external devices communicating through these ports. Programs utilizedby the CPU 48 are stored in a memory 50 which is comprised of bothvolatile and nonvolatile memory. The SCM cable 16 is connected to theinput of a single cable multiplexer (SCM) 51 that both transmits andreceives data on the SCM cable 16 and interfaces with a data port, avideo port and an audio port. There is a separate port for data input,video input and audio input and a separate port for data output, videooutput and audio output. As will be described hereinbelow, theaudio/video/data is modulated onto a first RF carrier for transmissionfrom the Mate 10 and demodulated from a second RF carrier for receivedaudio/video/data. The baseband information is input to the SCM 51 andtrasmitted over the SCM cable 16 at baseband for normal basebandcommunication.

The video signals from the VCR auxiliary device 28, the SCM 51 and theinternal camera 32 are input to a video switch 52 for selectiveconnection to the various outputs of the Mate 10. One switched output isconnected to the viewfinder 14, one output is connected to the videoinput of the SCM and another output is provided for connection to anauxiliary circuit labeled AUX. A fourth output is provided forconnection to the input of a decode circuit 56 which has its outputconnected through a switch 58 to the monitor 12. The decoder 56 isoperable to decode the video transmitted through the video switch 52into a format compatible with the operation of the monitor 12. In thepreferred embodiment, the monitor 12 operates in a "RGB" mode requiringonly information regarding the red, green and blue color levels with theaddition of horizontal and vertical sync, whereas the video switch 52 isoperable to process video in a format that complies with RS-170 NTSClevels. However, depending upon the decoder that is used, other types ofvideo can be processed by the video switch 52 as long as the viewfinder14 is compatible therewith. The switch 58 is operable to switch betweenthe local processor 42 and the decoder 56. Although the local processor42 could be processed by the video switch 52, the preferred embodimentutilizes the interface circuit 44 in FIG. 1 to reformat the output ofthe local processor 42 into the RGB format. Therefore, when video isreceived from the local processor 42, the switch 58 is activated toapply the appropriate video signals to the monitor 12. The video switch52 is controlled by the CPU 48.

An audio switch 54 is provided for receiving the audio output of the SCM51, a synthesized sound output from the CPU 48 labeled "Sound Gen", anauxiliary input labeled AUX and the audio output of the SSIB 18. Themicrophone 34 is also interfaced with one input of an audio switch 54through a voltage controlled amplifier (VCA) 59. The microphone 34 iscontrolled by a switch 55 which can selectively disconnect themicrophone 34 from the input of the VCA 59. A similar switch 53 isprovided for selectively disconnecting the SSIB 18 from the circuit. Themicrophone 34 and the SSIB 18 are never connected at the same time.

The audio switch 54 can route any of the inputs to either an auxiliaryoutput labeled AUX or to the audio input of the SCM 51. A third outputis connected to the input of a VCA 60, the output of which is connectedto the audio input of the SSIB 18. The output of the VCA 60 is alsoconnected through a mute switch 62 to the input of a speaker 64. TheVCA's 59 and 60 are controlled by the CPU 48 to determine the gain ofboth the input audio and the output audio. As will be describedhereinbelow, there is no cross coupling of audio in the Mate 10 during avideo conference. This allows the VCA's 59 and 60 to separately controlthe outgoing audio level and the incoming audio level, thus providinglocal control of the round trip audio path, as will be described in moredetail hereinbelow. The various connections of the audio switch 54 arecontrolled by the CPU 48.

Switched Rate Monitor

The monitor 12 is an analog input, 135 volt color RGB monitor that hasthe capability to accept two horizontal rates. It operates at a firstrate for displaying NTSC rate video, thus operating at a horizontal rateof 15.73426 Hz and a vertical rate of 59.94 Hz. This horizontal raterequires a horizontal blanking interval of 10.9 microsec. The verticalblanking interval is 1.27 msec. Although the rate matches standard NTSC,the video supplied to the monitor will be RGB plus composite sync,RS-178 levels.

The monitor 12 operates at a second rate to provide it with a capabilityof displaying computer generated data which could conceivably originatefrom a variety of different local processors or computers. Thesecomputers typically generate data with a horizontal rate from 15.73426Hz to 31.5 Hz with a vertical rate ranging from 45 Hz to 60 Hz. However,each different computer generates data at only one horizontal andvertical frequency, thereby only requiring one additional scanning ratein addition to the standard NTSC rate. The video supplied to the monitoris the same format as for the NTSC rate video; that is, RGB at theRS-170A level (1.0 volt peak to peak) but the sync could be separatehorizontal and vertical syncs.

NTSC video has a composite sync associated with it which is a compositeof horizontal and vertical sync. Computers usually have separatehorizontal and vertical outputs. In order to be able to display eithertype of data on the monitor requires the ability to use either type ofsync. A signal is output from the CPU 48 to control the type of syncthat the monitor 12 will receive. In one mode, composite syncs will beutilized and in the second mode, separate horizontal and vertical syncsare present on respective inputs.

There are five inputs associated with the video input of the monitor 12.Three of them are the RGB video inputs, one for red, one for green andone for blue, and the other two inputs are horizontal sync and verticalsync. When composite sync is utilized, it will be present on thevertical sync input and the horizontal sync input will be ignored.

NTSC video expects a ten percent horizontal and vertical overscan of thecathode-ray tube (CRT). Computer generated data frequently does notrequire overscanning of the CRT and, in fact, some data would be lost ifoverscanning were used. The monitor 12 provides a control line inputfrom the CPU 48 to allow selection of an overscanning mode whennecessary. The amount of overscanning is adjustable by an internalcontrol in the monitor 12. Overscan is defined as the raster beinglarger than the face of the CRT.

When switching from one video source to another in the Mate 10, therewill be a finite amount of time before the monitor acquires sync and thepicture stabilizes. This time is minimized, but it is not reduced tozero. To make the switch appear less obtrusive to the viewer, the videois ramped down quickly before the switch is made. The video is thenramped back up after a specified amount of time has elapsed. Thisprocess usually occurs in less than 0.5 seconds. A monitor which willprovide the switchable scanning frequencies is manufactured by VictorCompany of Japan, Ltd., part No. SD-H2114DP.

Mate Master/Slave Operations

In operation, the Mate 10 can operate in a Master or Slave mode. Itservices either the local processor 42, the SSIB 18 or the network. Inthe Master mode, the Mate 10 is utilized to select data and audio fromthe SSIB 18 and execute instructions in response thereto. The SSIB 18 isutilized to allow a user to enter keystroke instructions to the Mate 10to perform such functions as determining whether video should bedisplayed on a monitor 12 or the viewfinder 14 and deciding where thevideo is to be received, i.e., the local processor, the auxiliary port,the camera port or the network. Additionally, keystrokes can be input onthe SSIB 18 to input unique identifiers (ID's) of other remote terminalsor work stations for initiating a video conference therebetween, as willbe described hereinbelow.

In the Slave mode, the Mate 10 accepts input only from the localprocessor and passes through all other communications. It serves as theaudio and video input/output and communications terminal for theapplication program running in the local processor. In this mode, theMate passes messages received from the SSIB 18 to the local processorand only switches in response to receiving a command from the localprocessor. By providing for a separate local processor, a more versatileremote terminal is provided. This allows for a given remote terminal tobe compatible with already existing equipment in an office.

For example, if one office utilizes a local processing system of a firsttype and a second office utilizes a local processing system of a secondtype, both processing types can be interfaced to the Mate 10 merely byselecting a different interface circuit 44. In this manner, both thetype and the size of the local processor 42 can vary and still beaccommodated by the Mate 10. When utilized with a separate localprocessor 42, the Mate 10 provides the necessary processing capabilityto further extend the versatility of the remote terminal, in that theMate 10 enables the local processor 42 to interact with the network.However, only the Mate can initiate and maintain a communication linkwith the network. The local processor 42 cannot directly communicatewith the network but, rather, it requests the Mate to communicate forit.

First Level Conferencing Network

Referring now to FIG. 3, there is illustrated a diagrammatic view of thesimplest form of a video conferencing network which is designated as aFirst Level Conferencing Network. This network is comprised of a centralswitching network 66 referred to as a Multimedia Information Exchange(hereinafter referred to as Mix). The Mix 66 has six ports connected tosix Mates 10 labeled "01", "02", "03", "04", "05" and "06" throughassociated SCM cables 16. However, the Mix 66 could be connected toadditional Mates 10, depending upon the internal configuration thereof.In the preferred embodiment, the Mix 66 is interfaced with eight Mates10, but for simplicity purposes, only six Mates 10 are illustrated.

Internally the Mix has a full nXn crosspoint switch for the audio andvideo paths of each port, n being equal to the number of Matesinterfaced with the Mix. Each switch is independently controllable toprovide nonblocking operation. A serial data control path is associatedwith each Mate 10 and these are routed independently to a controlprocessor internal to the Mix 66.

Two Way Conference

In operation of the configuration shown in FIG. 3, the Mix 66 is incommunication with each of the Mates 10 on the network through a serialdata path in the SCM cables 16. Upon receiving a request from any of theMates 10 to place a call to another Mate in the network, the Mix 66determines if that Mate is available and, if so, "places the call." Thecall is placed by providing a video and an audio path between the twoMates 10 in the conference. The basic procedure for placing a call is tofirst send a message to the Mix 66 requesting a call. This messagecontains information regarding both the ID of the originating Mate andalso the ID of the destination Mate and is transmitted along the datapath. The Mix 66 then acknowledges to the originating Mate that it hasreceived this information and then determines if a path is available tothe destination Mate. This is determined from an internal status tablewhich the Mix 66 maintains such that the status of all Mates on thenetwork is known.

If a line is available, the Mix 66 then determines if the destinationMate is "busy". If so, the originating Mate is notified by anappropriate message. If the destination Mate is not busy, a message issent from the Mix 66 to the destination Mate indicating that there is anincoming call. Prior to sending of the incoming call message, the switchconnection is made to provide a video and audio path between theoriginating and destination Mates. The destination Mate then provides aringback signal to the originating Mate to indicate that it is locallyoutputting an audible ring or "chime". When the call is received byinputting an appropriate key stroke at the destination Mate, video andaudio is received by the destination Mate and, simultaneously, video andaudio is transmitted from the destination Mate. The video and audio pathprovided by the SCM cable 16 and the Mix 66 is a full duplex two-waypath wherein both paths are isolated such that there is nocross-coupling between video or audio. This isolated two-waytransmission is provided by a frequency division multiplexer, as will bedescribed hereinbelow.

Multi-Way Conference

To connect more than two Mates in the system shown in FIG. 3 together ina video conference, it is necessary for an originating Mate to firstplace a call with a destination Mate and then place this destinationMate on hold and contact another Mate. For example, if Mate 01 wishes toset up a video conference with Mates 04 and 05, a call is first placedto destination Mate 04, as described above. A hold signal is thentransmitted from the originating Mate 01 to the Mix 66. Upon receipt ofthis hold signal, a message is sent to destination Mate 04 to place iton hold. Destination Mate 04 then "loops" back the internal video suchthat the party at the destination Mate 04 receives his own videotransmission. A call is then placed to destination Mate 05 and a two-wayconference initiated. This two-way conference between originating Mate01 and destination Mate 05 is maintained and destination Mate 04maintained on hold until a message is sent from the originating Mate 01to the Mix 66 to bring all the parties back into the conference. Aconference table is maintained internal to the Mix 66 to determine whichMates are a part of the conference.

During a video conference, each Mate receives audio from all of theother Mates in the conference, this audio being summed at the Mix 66.Each Mate's own audio is not summed to prevent feedback. However, onlyone video transmission can be made from the Mix 66 to any of the Mates.The video that is displayed on any of the monitors 12 of the Mates 10 inthe conference is determined by a priority system, which will bedescribed in detail hereinbelow.

Mix Switching Network

Referring now to FIG. 4a, there is illustrated a block diagram of theMix 66 of FIG. 3. Each of the input ports are connected to one of theSCM cables 16 and are interfaced to a separate SCM module 68 labeled"SCM 01" through "SCM 08", corresponding to Mates with ID numbers 01through 08. Since, as described above, the preferred number of Matesassociated with each Mix is eight, the Mix of FIG. 4 is illustrated asbeing interfaced with eight SCM's 68. The SCM's 68 are similar to theSCM 51 of FIG. 1 in that they strip off the baseband information anddemodulate the received audio/video/data and, for transmission, modulateaudio/video/data and sum this with baseband information.

Each of the SCM's 68 provides a data output and a data input, a videooutput and a video input, an audio output and an audio input and abaseband bidirectional input. The audio inputs and outputs of the SCM's68 are connected to the inputs of an audio switch/summer 70. The audioswitch/summer 70 receives eight inputs and provides eight outputs, allof which can be selectively summed with each other. The video outputsand inputs of the SCM 68 are interfaced with a video switch 72 which iscontrolled to selectively switch video between the SCM's 68. However,the video switch 72 does not sum the videos such that only one of thevideo inputs to the video switch 72 can be connected to any one of thevideo outputs at any given time. However, one video input can beconnected to all eight outputs. As a practical consideration, videowould only be output to a maximum of seven outputs since the originatorof the video would not require its video to be fed back.

The Mix 66 has a bidirectional serial data port and an auxiliary serialreceiver/transmitter circuit 74 (AUX) for receiving and transmitting theserial data. This is utilized to interface with a serial format such asRS-232 in processing data. In addition, a local area network (LAN) datainterface circuit 76 is provided for receiving baseband data from andtransmitting data to a LAN network. The received baseband data isprocessed by the LAN circuit 76 and placed in a format for internal useby the Mix 66 whereas data transmitted from the LAN data interface 76 isconverted to baseband data. Although the LAN data interface 76 isillustrated as being connected to a baseband LAN, it should beunderstood that any type of LAN format can be accommodated. The LANcircuit 76 is utilized only to provide a data interface between acentral processing unit and the Mix for control data. Therefore, the LANcircuit 76 forms a ninth port. However, this ninth port is only a dataport and does not have video or audio associated therewith. From thestandpoint of data communications, the LAN circuit 76 operates similarto the SCM 68 in that it terminates one of the ports of the Mix 66 andallows data communication between two of the devices.

Each of the SCM's 68 have a baseband port connected to a common node 77.This common node 77 interconnects all the baseband ports such thatbaseband data transmitted on one SCM cable 16 is interfaced with all ofthe other baseband information on the other SCM cables. As describedabove, the baseband data transmitted on any of the SCM cables 16 doesnot carry control data for the Mix or for the video conferencingnetwork. Rather, the baseband link provided through the SCM cable 16merely facilitates integration of the video conferencing system with analready existing baseband system. However, this is to be distinguishedfrom the baseband interface provided by the LAN circuit 76. This is adedicated data path for control data generated for the videoconferencing system only.

A CPU 78 is provided for controlling the operation of the Mix 66. Boththe switching and summing operations in the audio switch/summer 70 andthe switching operation of the video switch 72 are controlled by the CPU78. In addition, the CPU 78 interfaces with data from the auxiliaryserial circuit 74 and the LAN interface circuit 76. To provide data forthe serial data path to the Mate 10, a serial receiver/transmitter 80 isprovided which is connected to the data inputs and outputs of the SCMs68. Memory 82 is provided for interfacing with the CPU 78 to provide astorage base for the operational software of the Mix 66.

First Level Network Data Flow

In operation of the system shown in FIG. 4a, each of the Mates 10 thatare attached to Mix 66 have a direct serial data path with the Mix 66.The Mix 66 provides two functions. First, it operates as a switch tocontrol both the audio switch 70 and the video switch 72 to provide anaudio and video path for conferencing between Mate 10 in the system.Secondly, it provides control for the conferencing network to store thestatus of all of the Mate 10 in the system and also the status of allconferences. To maintain this status, the Mix 66 continually polls thestatus of all Mates on its various ports. This provides the Mix 66 withinformation regarding the ID of the Mate and the port to which it isattached and also the status thereof, such as "busy", etc. There is nodata path between two Mates in the system but, rather, all data flowmust go directly to the Mix and be processed thereby. For example, ifthe Mate 10 is placing a call, it must request the Mix 66 to check thelines to determine if there is a path available and also to determine ifthe destination Mate is available. If available, the Mix 66 then sends amessage out to the destination Mate to initiate the generation of anaudio and video path between the two Mates. Even during a conference,the Mix 66 continually polls the Mates in the conference to determinetheir status with no data flow occurring between any of the Mates.

Referring now to FIG. 4b, there is illustrated a diagram for thetransmission path of both the audio/video/data information for the videoconferencing mode and also baseband data for communication between thelocal processor baseband output and the separate Local Area Network(LAN) through the SCM cable 16, wherein like numerals refer to likeparts in the various Figures. The SCM 51 in the Mate 10 is illustratedas having two video information lines 61, one for inputting video intothe SCM 51 and one for receiving video from the SCM 51. Two audio inputlines 63 are illustrated for inputting audio into the SCM 51 andreceiving audio therefrom. Two serial data lines 65 are illustrated forinputting serial data into the SCM 51 and receiving data therefrom. Inaddition, a baseband input 67 is illustrated that is received from thelocal processor 42. As described above with reference to FIG. 1, thisinput is received from the local processor 42 on the line 47 and thiswould normally constitute a direct interface between the local processor42 and a separate LAN in the absence of any video conferencing network.

The SCM 51 is connected to the SCM 68 in the Mix 66 through the SCMcable 16. The SCMs 51 and 68 provide three separate and distinct pathsfor data transmission. A first path 69 is provided for transmission ofaudio, video and data from the Mate 10 to the Mix 66 for videoconferencing purposes. A data transmission path 71 is provided fortransmission of audio, video and data from the Mix 66 to the Mate 10,also for video conferencing purposes. A third separate and distinct path73 is provided for transmission of baseband data that is expressly forthe purpose of interfacing with the separate LAN operating at baseband.There is no cross coupling between the data transmitted over the path 73which is a baseband data path or the audio/video/data information on thepaths 69 and 71. In a similar manner, the paths 69 and 71 are alsoisolated from each other. This isolation between lines 69 and 71provides the full duplex operation of the video conferencing network ofthe present invention.

The SCM 68 in the Mix 66 has two lines 75 that are input to the videoswitch 72, one for outputting video information from the SCM 68 and onefor inputting video information thereto. Two lines 79 are provided forinterfacing with the audio switch 70, one for receiving audioinformation from the SCM 68 and one for inputting audio informationthereto. Two data lines 81 are provided for interfacing with the serialreceiver/transmitter 80, one for receiving data from the SCM 68 and onefor transmitting data thereto. The lines 75, 79 and 81 are all internalto the Mix 66 and interface with the various internal componentsthereof. However, the SCM 68 also filters off the baseband informationthat was contained on the data path 73 and outputs it on the line 77 tothe separate LAN. Therefore, the Mix of FIG. 4a with eight SCMs 68 wouldhave eight lines 77 coming from the Mix 66 to the separate LAN. The SCMcable 16 merely provides a shared data path up to the Mix 66. However,at the Mix 66, the baseband information to the LAN is filtered off andoutput to a separate cable for connection to the LAN. Wheninterconnecting a number of cables, it is necessary to provide impedancematching and isolation thereto. Therefore, all of the local processors42 interfaced with the various Mates 10 can utilize the SCM cable 16 totransmit baseband information to the LAN and carry on datacommunications apart from the video conferencing network. It isimportant to note that the operation of this LAN is separate anddistinct from the video conferencing network and the data transmittedover the path 73 is utilized as a data path from the LAN to the localprocessor and not to the Mix 66. The LAN interface from the LAN to thevideo conferencing network must occur through the LAN data interface 76,as will be described hereinbelow with reference to control of the Mix 66by a centrally located processor on a LAN.

Referring now to FIG. 5, there is illustrated a graph of the frequencyspectrum allocation for the SCM cable 16. The lower portion of thespectrum is denoted by reference numeral 84 and is allocated totransmission of the baseband data and is primarily dedicated to use bythe local area network apart from the video conferencing network. Theaudio/video/data for the video conferencing network occupies twoportions of the spectrum, a portion 86 and a portion 88. The spectrumportions 86 and 88 are wideband FM modulated signals with the portion 86centered around 70 Hz and the portion 88 centered around 170 Hz. Theaudio/video/data information contained in the spectral portion 86 isutilized for transmission in only one direction on the SCM cable 16 andthe audio/video/data information contained in the spectral section 88 isutilized for transmission in the opposite direction on the SCM cable 16.

In this manner, frequency division multiplexing is utilized to providetwo distinct data paths in a single cable which are isolated from eachother. Therefore, the audio/video/data passing from the Mate to theassociated Mix is isolated from the audio/video/data transmitted fromthe Mate to the Mix. Additionally, as described above, baseband data canbe transmitted on the same cable to allow the network to accommodate aseparate LAN. The baseband is separated off at the Mate and output tothe local processor or other LAN device. It is not necessary for the LANdevice at the Mates location to interface with the Mate other than thebaseband interconnect.

Single Cable Multiplexer

Referring now to FIG. 6, there is illustrated a block diagram of thesingle cable multiplexer circuits 68 in the Mix 66 and the single cablemultiplexer circuit 51 in the Mate 10. The SCM cable 16, as describedabove, is a single coaxial cable which is connected to an input jack 90on either the Mix 66 or the Mate 10. The jack 90 is connected directlyto one port of a broadband low-pass filter 92, the other port of whichis interfaced with a LAN device. The broadband low-pass filter 92isolates the wideband frequency modulated signal from the LAN device.The jack 90 is also input to one port of a high-pass filter 94. Theother port of the high-pass filter 94 is interfaced with a modulationcircuit 96 and a demodulation circuit 98. The modulation circuit 96receives video, audio and data on separate ports thereof and modulateseither a 70 mHz or a 170 mHz carrier frequency with this data. Dependingupon whether the SCM circuit is associated with either the Mate or theMix, the modulated carrier will vary. For example, if the modulatedcarrier for transmitted data is 70 mHz in the Mix, the transmitted dataon the Mate will be at 170 mHz to maintain isolation between the twopaths.

The demodulator 98 receives the output of the high-pass filter 94 anddemodulates the audio and video data. The demodulator circuit 98operates at the opposite frequency from the modulator. Therefore, if themodulation circuit 96 is operating at 70 mHz, demodulator circuit 98 isoperating at 170 mHz. The demodulator circuit 98 provides a data output,an audio output and a video output on separate lines.

The modulation circuit 96 receives the data and modulates it with amodulator 100 onto a subcarrier of 9.0 mHz and also receives the audioand modulates it onto a subcarrier of 10.7 mHz with a modulator 102. Themodulated subcarriers output by the modulators 100 and 102 and the videoare then summed and input to a 70/170 mHz wideband FM modulator 104. Theoutput of the modulator 104 is input to a 70/170 mHz bandpass filter 106which has a bandwidth sufficient to pass the spectral information thatis output by the modulator 104. This information is passed through thehigh-pass filter 94 to the jack 90 for input to the SCM cable 16.

The demodulation circuit 98 processes the output of a high-pass filter94 through a bandpass filter 108 to reject the spectral output of themodulation circuit 96. The bandpass filter 108 is centered around either170 Hz or 70 mHz, the opposite frequency from that of the modulationcircuit 96. The output of the bandpass filter 108 is input to a 170/70mHz wideband FM demodulator 110 which provides demodulated video and thetwo modulated subcarriers for audio and data. The output of thedemodulator 110 is input to a bandpass filter 112 that is centered aboutthe data subcarrier of 9.0 mHz and then demodulated by a demodulationcircuit 114 to output data. A bandpass filter 116 is centered about theaudio subcarrier of 10.7 mHz and the signal output therefrom isdemodulated by a demodulator circuit 118 to provide the audio output.

The modulators 100 and 102 are FM modulators and are fabricated fromdevices such as the MC1376 manufactured by Motorola Corp. However, anytype of modulator can be utilized. The summing circuit for adding themodulated audio, modulated data and video can be an operationalamplifier of the type HA-5195 manufactured by Harris Semiconductors,Inc. The demodulation circuit 110 utilizes phase lock loop technologyfor wideband FM demodulation. The demodulator 114 for demodulating datais merely a quadrature detector whereas the demodulator 118 fordemodulating the audio is of the type MC3356 manufactured by MotorolaCorp. Although not shown, an auto symmetry correction circuit isprovided for restoring the baseline for the data. A similar type circuitis illustrated in U.S. Pat. No. 4,309,763, issued to Passmore, et. al.on Jan. 5, 1982.

Second Level Video Conferencing Network

Referring now to FIG. 7, there is illustrated a video conferencingnetwork having a plurality of Mix networks contained therein. Thenetwork is comprised of a Mix 120 labeled "01", a Mix 122 labeled "02",a Mix 124 labeled "03" and a Mix 126 labeled "04". The labels "01"through "04" are defined as the ID's of the Mixes in the network. It isnecessary for each Mix to have an ID to determine priority of networkcontrol, as will be described hereinbelow. In the single Mix networkdescribed above with reference to FIG. 3 and the First Level VideoConferencing Network, the single Mix controls all network functions,audio/video interconnects, etc. By comparison, the Second Level Networkdistributes the control in a predetermined manner.

The Mixes 120-126 for simplicity purposes are illustrated as having sixports. The Mix 120 interfaces with the Mix 126 through an SCM cable 128,with the Mix 124 through an SCM cable 130 and with the Mix 122 throughan SCM cable 132. The Mix 126 interfaces with the Mix 122 through an SCMcable 134 and with the Mix 124 through an SCM cable 136. The Mix 124interfaces with the Mix 122 through an SCM cable 138. It is necessaryfor each Mix to directly interface with every other Mix in the networkwithout having to pass through another Mix. Therefore, depending uponthe number of ports in a given Mix, the number of Mixes in a givennetwork is limited since each additional Mix occupies an additional porton every other Mix in the network, thereby reducing the number of remoteterminals that can be interfaced with each Mix.

The Mix 120 has a Mate 140 interfaced thereto through an SCM cable 142,a Mate 144 interfaced therewith through an SCM cable 146 and a Mate 148interfaced therewith through an SCM cable 150. The Mix 122 has a Mate152 interfaced therewith through an SCM cable 154, a Mate 156 interfacedtherewith through an SCM cable 158 and a Mate 160 interfaced therewiththrough an SCM cable 162. The Mix 126 has a Mate 164 interfacedtherewith through an SCM cable 166, a Mate 168 interfaced therewiththrough an SCM cable 170 and a Mate 172 interfaced therewith through anSCM cable 174. The Mix 124 has a Mate 176 interfaced therewith throughan SCM cable 178, a Mate 180 interfaced therewith through an SCM cable182 and a Mate 184 interfaced therewith through an SCM cable 186.

In operation, each of the Mixes 120-126 has a Master mode and a Slavemode for transferring data through the network. In the Slave mode, theMix services its own ports to determine the status of all of the deviceson each of the ports. This information is maintained in internal tablesand defines the status of each of the Mates interfaced therewith andalso the status of each of the Mixes interfaced therewith. For example,Mix 122 knows that there are three Mates 152, 156 and 160 attached tospecific ports and the ID's of each. In addition, it knows that Mixes120, 124 and 126 are attached to specific ports and the ID's of each.Mix 122 constantly polls the devices attached to its ports to determinetype of device, ID and status.

In the Slave mode, one Mix in the network does not directly interfacewith a Mate on another Mix in the network. In the Master mode, whichoperates in conjunction with the Slave mode, the Mix controls theswitching operation of all the Mixes in the network, services all of thenetwork requests from the Mate and the other Mixes and also controlsmultiple Mate video conferences. However, only one Mix in the networkcan operate in the Master mode. Of the Mixes 120-126 in the network ofFIG. 7, only the Mix 120 with the ID "01" is allowed to operate in theMaster mode. This is a predefined priority wherein the Mix with thelowest ID in the network operates in the Master mode. If the Mix 120were turned off and removed from the network, the Mix 122 with the ID"02" would then be reconfigured in the Master mode. In this situation,the network would have to "reconfigure" to update the internal tables inthe Master Mode of the Mix 122. The Mix that is operating in the Mastermode is termed the " Network Master".

To process a simple call in the network, one of the Mates, for exampleMate 140, places a request to call on the 9600 baud serial data path tothe Network Master which, in this configuration, resides in Mix 120. TheNetwork Master then looks up in its table to determine both if there isa path available and the shortest path to the destination Mate. It thendetermines if the destination Mate is busy and, if not, it sends anincoming call message to the destination Mate to place the call. Oncethe call is placed, the Network Master then reconfigures its internaltables to reflect the new communication path. Additionally, the NetworkMaster can also determine if a previous path has been abandoned andpossibly reconnect an ongoing call to this abandoned path if it is ashorter path. Since the transmissions through the network are analog, itis necessary to minimize the signal to noise ratio, thereby requiringthe shortest path for communication.

The Mixes 120-126, when operated in the Slave mode, periodically checkon the status of all of the ports with "keep alive" messages. In thismanner, the status of all the devices can be maintained in an internaltable. If the status changes by either adding a call or deleting a call,the Mix in the Slave mode denotes this and sends out a Reconfiguremessage to the Network Master. The Network Master then acknowledges andthe reconfiguration information is then passed from the Slave to theNetwork Master to maintain an updated table in the Master. Each Mix inthe network knows which port the Network Master is connected to sincethe lowest ID constitutes the Network Master. When the Network Master isremoved from the network, it is then necessary for a new Network Masterto take over and update its table. This would be the next lowest ID ofthe Mixes 120-126 remaining in the network.

Second Level Network Data Flow

Referring further to FIG. 7, messages are transmitted on the serial datapath from the Mate to either its associated Mix in the Slave mode or tothe Network Master, from a Slave Mix to either the Network Master or oneof its associated Mates or from the Network Master to any of the SlaveMixes or to any of the Mates. A Mate cannot send data to another Mate orSlave Mix other than its associated Slave Mix nor can a Slave Mix senddata to a Mate attached to another Slave Mix. However, a Slave Mix cansend a message to the other Slave Mixes for status information. Whenplacing a message onto the serial data path, the originating deviceencodes this message with information regarding the ID of theoriginating Mate and also the ID of the destination Mate. For example,if Mate 152 sends a System message out to the Network Master, it encodesits unique ID as the originating device and the ID of the Network Masteras the destination device. The Network Master has an ID that is definedas "00". This message is transmitted to the Mix 122 which is operatingin the Slave mode. The Mix 122 recognizes the signal as being directedtoward the Network Master, which in this example is Mix 120. Thismessage is not processed by the Mix 120 in the Slave mode but, rather,routed through the SCM cable 132 to the mix 120. Mix 120 recognizes thesignal as being directed toward the Network Master and also recognizesthat the Network Master resides internal thereto. The message is thenprocessed by Mix 120 in the Master mode.

After receiving the message from Mix 152, Mix 120 in the Master modetransmits a response back to Mix 152. This message is also encoded withthe ID "00" as the originating device and the ID of the Mate 152 as thedestination device. Mix 120 first recognizes that the destination Mateis not attached to any of the ports but recognizes that it is directedtoward a Mate on one of the ports of Mix 122. The data is thentransmitted back along the SCM cable 132 to the Mix 122. The Mix 122recognizes this message as being directed toward the Mate 152 ratherthan to itself and routes the data thereto. Therefore, the Mixes in theSlave mode do not process data from the Network Master prior to routingit to the destination Mate but, rather, access the data, examine thedata and then reroute the data.

In another example, the Mix 122 can directly communicate with theNetwork Master residing in the Mix 120. In this example, the Mix 122generates a message having its ID as the originating device and theNetwork Master ID as the destination device. The message is thentransmitted from the Mix 122 to the Mix 120, recognized by the Mix 120as being for the Network Master and allowing the internal network masterto process the data. As will be described hereinbelow, the NetworkMaster comprises a virtual tenth port internal to the Mix in which it isresiding. Therefore, the data is accessed by the associated Mix and thenrouted to this virtual tenth port.

Third Level Video Conferencing Network-LAN Based Central

Referring now to FIG. 8, there is illustrated an alternateinterconnection between single Mix networks using a broadbandinterconnect system. The network is fabricated from a plurality of Mixnetworks of which three are illustrated. A Mix 188 is labeled with theID "01", a Mix 190 is labeled with the ID "02" and a Mix 192 is labeledwith the ID "XX". The Mix 188 has four Mates 194 associated therewith,the Mix 190 has four Mates 196 associated therewith and the Mix 192 hasfour Mates 198 associated therewith. There can be a plurality of Mixnetworks with associated Mates in the illustrated network, each having aspecific ID.

The Mix 188 is interfaced with a local area network (LAN) 200 through aLAN interface circuit 202 to allow data transfer therebetween. In asimilar manner, the Mix networks 190 and 192 are interfaced with the LAN200 through LAN interfaces 204 and 206, respectively. The LAN interfaces202-206 are, in the preferred embodiment, integrated into the Mix, asdescribed above with reference to the LAN interface 76 of FIG. 4. Theyallow the onboard CPU in the Mix to communicate with the LAN. Interfacedwith the LAN 200 is a central processing unit 208, hereinafter referredto as the "Central". Whenever the Central 208 is operable, each Mix inthe network recognizes the Central 208 as the Network Master regardlessof the ID's of the Mixes. Therefore, each of the Mixes in the videoconferencing network of FIG. 8 is provided with a direct data interfaceto the network Master. However, if the Central 208 is removed, each ofthe Mixes operates independent of the remaining Mixes in the broadbandconfiguration of FIG. 8. It is important to note that the LAN 200 isoperable to carry control data at baseband between Mixes in the Networksince it is interfacing directly to the internal CPU of each Mix throughthe interface circuits 202-206. This is to be distinguished from thebaseband data transmitted on the SCM cables between Mates and theassociated Mix. For example, baseband data transmitted directly from thelocation of one of the Mates 194 does not get transmitted to the LAN200. The LAN 200 is only for network data in the configuration of FIG.8.

The Mix 188 is interfaced with a broadband cable network 210 through anaudio and video modulator/demodulator (modem) 212 which isinterconnected with one of the ports thereof. In a similar manner, Mix190 and Mix 192 are also interfaced with the broadband network 210through audio/video modems 214 and 216, respectively. The broadbandnetwork is a network such as a CATV system which provides a plurality ofdiscrete channels which are selectable by the audio/video modems212-216. Selective switching of channels with the modems 212-216 enablesa data link to be formed between any two of the Mixes 188-192. Once achannel in the broadband cable network 210 is selected by the modemsassociated with Mixes in a video conference network, the data linkappears no different than the SCM cable link in the networkconfiguration of FIG. 3. There is still a direct audio/video/data pathbetween interconnected Mixes for a two way conference. However, the useof the LAN for communication between the Network Master and associatedMixes in the Slave mode eliminates the need for transmitting data overthe broadband network 210.

When the system of FIG. 8 is utilized for multiway conferences, theNetwork Master in Central 208 must control the modems 212-216 to selectthe proper channels such that the correct video and audio is beingselected. To properly route the video, a priority is established, aswill be described hereinbelow. This priority essentially designates oneuser as the Primary whose video output is routed to all of the remainingMates in the conference. The Mates on different Mixes are in theconference, it is then necessary that the modems 212-216 all have thevideo portion thereof tuned to that of the Primary. In addition, theprimary user who is transmitting video to the remaining confereesreceives video from only one of the conferees and this conferee isdesignated as the secondary. Therefore, the associated modem must betuned to the video from the secondary if the secondary video istransmitted over the broadband network 210.

In one example, one of the Mates 194 is the Primary and one of the Mates196 is the secondary and the M-ary's are comprised in part of the Mates198. The broadband network 210 has channels A-Z with the modems 212-216operable to select any of these channels for transmission thereover. TheNetwork Master would predesignate channel A as having video from theprimary transmitted thereover such that both the secondary Mate 196 andthe M-ary Mates 198 would receive video therefrom. The Network Masterwould also designate channel B for transmission of secondary video.Therefore, the video transmitting portion of the modem 212 would betuned to channel A from the Primary Mate 194 and the video receivingportion of the modem 212 would be tuned to channel B for receivingvideo. The modems 214 and 216 would have the video receiving portionthereof tuned to channel B with the modem 214 having the videotransmitting portion thereof tuned to channel B for transmission ofvideo to the Primary Mate 194. In this manner, it is only necessary tohave one video transmission path between the primary and all theremaining M-arys and one video transmission path between the Secondaryto the Primary.

When the audio from Mates on different Mixes is involved in a multiwayconference, it is necessary to sum all of the audio for reception byeach of the Mates in the system with the exception of its own audio.Therefore, it is necessary to have a direct or virtually direct audiopath between each Mate in the multiway conference and all the remainingMates therein. To facilitate this, the audio portion of each modem istuned to each channel. In the above example, the audio output from eachof the Mixes 188-192 would be transmitted on a dedicated channel.However, the modems 212-216 are operable to receive audios from all ofthe channels simultaneously and sum this audio for transmission to theassociated one of the Mixes 188-192. In this manner, each of the Matesin the conference can hear the summed audio from all the remaining Mixestherein.

For example, if two of the Mates 194 on the Mix 188, two of the Mates196 on the Mix 190 and two of the Mates 198 on the Mix 192 wereassociated in a multiway conference, the audio portion of the modem 212would be set for audio transmission on channel A, the modem 214 set tohave audio transmission on channel B and the audio/video modem 216 setto have audio transmitted on channel C. The Mixes 188-192 would sum theaudio from the associated Mates 194, 196 and 198, respectively andoutput it to the respective modem 212-216. The modem 212 would be set toreceive the audio from both channel B and C, the audio portion of themodem 214 would be set to receive audio from channels A and B and theaudio portion of the modem 216 would be set to receive audio fromchannels A and B. This would effectively allow each of the Mates to hearthe remaining Mates in the conference.

If the LAN were not utilized for a data interface between the Mixes inthe broadband system, each of the Mixes 188-192 would then require adedicated full duplex data interface with each other. For example, ifonly three Mix networks were utilized, each would require two dedicatedchannels for a data interface between the remaining Mixes in thenetwork. This would require multiple modems, one modem for each pair ofchannels.

In operation, the Central 208 operates as a separate and distinctprocessor on the LAN 200 and each of the LAN interfaces 202, 204 and 206also operates as a separate and distinct device attached to the LAN.Therefore, conventional LAN protocol would be utilized for communicationbetween the various devices. For example, if Central 208 desired tocommunicate with the Mix 188 through the LAN interface 202, it wouldutilize the appropriate protocol to address the LAN interface 202 andtransmit data thereto. If another Mix 190 desired to communicate withthe Central 208 at the same time, it would have to follow the protocolof the LAN 200 in order to transmit the data to the Central 208. Sincedata is transmitted between the Network Master operating in the Central208 by sending a message and then waiting for a response, slight delaysmay be incurred in communicating with the Central 208 when a largenumber of Mixes are connected to the buffer.

Although the LAN 200 in FIG. 8 is illustrated as being dedicated to theCentral 208 and the associated Mixes 188-192, it is also possible thatthe LAN 200 can accommodate other peripheral LAN devices. For example,the local processors that are interfaced with the Mates on the variousMixes 188-192 could also be directly connected to the LAN 200 andcommunicate with each other through the LAN along a direct basebandpath. As described above with reference to FIG. 4b, a local processorwould send baseband data up through the SCM cable 16 to the receivingSCM 68 at the Mix and this baseband data would be stripped off and thendirectly connected up to the LAN 200. In a similar manner, baseband datafrom the LAN 200 would be directly connected to one of the SCMs 68 inone of the Mixes 188-192 through a separate cable and then transmitteddown the SCM cable 16 to the receiving local processor. It is importantto note that communication from a device on the LAN to one of the localprocessors addresses the local processor directly. This is to bedistinguished from communication with the video conferencing networkwherein one of the LAN interfaces 202-206 would have to be addressed andthe data transmitted thereto and converted into serial data compatiblewith the associated Mix.

Local area networks are distinguished by the area they encompass. Theyare geographically limited from the distance of several thousand feet toa few miles and are usually confined to a building or a plant housing agroup of buildings. In addition to its local nature, the LAN hassubstantially higher transmission rates then networks covering largeareas. Typical transmission speeds range from 1 mbit/s to 30 mbits/s.LAN's do not ordinarily include the services of a common carrier but aremore commonly privately owned and operated, thus avoiding theregulations of the FCC or the State Public Utility Commission. LAN's areusually designed to transport data between computers, terminals andother devices. Some LAN's are capable of video signaling as well. LAN'semploy many techniques to manage data flow such as switching, digitizingschemes, data link control, modulation and multiplexing. LAN's arecomprised of a number of basic components. The most important componentis the communication path which most often consists of coaxial TV cableor a coaxial baseband cable. Cable TV (CATV) coax is used on manynetworks because it has a high capacity, very good signal to noiseratio, low signal radiation and low error rates. Twisted pair cable andmicrowave pads are also found in many LAN's. Baseband coax is anotherwidely used transmission path, giving high capacity as well as low errorrates and low noise distortion.

The second item that is a part of the LAN is the interface between theuser and the network. This interface can take the form of a modem fortransferring data onto a broadband network or a baseband interfacecircuit for transferring data in baseband onto a baseband cable. Thethird item is a protocol control which contains a control logiccomponent for the LAN and provides for the user's access onto thenetwork. Most LAN protocols employ methods and techniques forcontrolling data accountability and line control. These involve varioustypes of polling/selection techniques, error handling techniques, etc.One form of protocol, which is utilized in the preferred embodiment, istoken passing which is a ring topology where a token or a time slot ispassed to the next station on the ring network. The last major componentis the user workstation which can be anything from a word processor to amainframe computer.

In utilizing a LAN, each of the devices that is interfaced with the LANmust abide by the network protocol in order to transfer information backand forth between units. When a LAN is utilized only with the videoconferencing system, such as that shown in FIG. 8, the only informationtransmitted over the LAN is control data between the Central 208 and theMixes associated therewith. The LAN interface modules 202-206 providethe necessary protocol to interface with the LAN, which protocolcomplements the operation of the Central 208. Therefore, the input ofeach of the LAN interface modules 202-206 is virtually the same as ifthe Network Master resided in another Mix and the data were being passedthrough an SCM cable. The Mix does not care where the Network Master isas long as it has a direct data path to the Network Master and it knowson which port the Network Master resides.

For example, if a Mix sends a request to the Central 208 wherein theNetwork Master resides, the message is sent out to the associated LANinterface and then the Mix waits for an acknowledgement. During thisperiod, the LAN interface, in accordance with the LAN protocol,transmits the message to the Central 208, Central 208 processes themessage and then sends an acknowledgement back on the LAN 200. Thelength of duration for this operation is dependent upon the speed of theLAN and the number of devices attached to the LAN. For the situationwhere the LAN is dedicated to video conferencing, this is much fasterthan processing serial control data through the SCM cables in a totalMix network. However, it is important to note that speed is the onlydifference with respect to the operation of the Mix. The interfacebetween the Mix which operates in the Slave mode and the Central whichcontains the Master task is virtually the same for any configuration.

Referring now to FIG. 9, there is illustrated an alternateinterconnection for the broadband network of FIG. 8 wherein likenumerals refer to like parts in the two Figures. The system of FIG. 9utilizes the broadband network 210 to provide the LAN data link. TheCentral 208 and the LAN 200 are interfaced with the broadband network210 through a LAN modem 218. The Mix networks 188-192 are interfacedwith the broadband network 210 through LAN modems 220, 222 and 224,respectively. The LAN modems 218-224 are connected to a fixed channel inthe broadband network 210 that is dedicated to communications betweenthe network and the Central 208, such that a data path is alwaysavailable without the requirement to switch between different channels.The operation of the system of FIG. 9 is identical to the operation ofthe system of FIG. 8 with the exception that the data travels throughthe broadband network 210 between the Central 208 and the Mixes 188-192.

Long Haul Data Link

Referring now to FIG. 10, there is illustrated a network configurationfor linking two independent video teleconferencing systems. In thesystem of FIG. 10, two First Level networks are illustrated, eachutilizing a single Mix with a cluster of Mates disposed therearound. Onenetwork is comprised of a Mix 221 with associated Mates 223. The secondnetwork is comprised of a Mix 228 with associated Mates 230. The Mix 221interfaces with a high frequency data link receiver/transmitter 225through a data link interface 226. In a similar manner, Mix 228interfaces with a high frequency data link receiver/transmitter 232through a data link interface 234. The interface 226 provides a paththrough which the Mix 221 can establish a data link with anotherreceiver/transmitter in the same network such as thereceiver/transmitter 232. In this manner, an off-network data link canbe made to another Mix. This data link is referred to by a referencenumeral 236.

The data link 236 can have the audio/video/data information transferredthereover such that the data link 236 and the interfaces 234 and 226appear as a virtual coaxial cable to the two Mixes 221 and 228. For thetwo Mixes 221 and 228 to interact as a common system, the delaysexperienced over a long haul data link such as the data link 236 must betaken into consideration to provide a feasible data interface.Otherwise, each Mix 221 and 228 will typically act independently of theother such that an integrated network is not formed but, rather, a datalink between two independent networks. For example, if a one-waysatellite data link were made, the up link and down link signals for aone-way transmission would require a minimum of approximately 240 mspropagation time (22,300 miles/186,000 miles per second=0.120×2=0.240ms) and it is most often around 270 ms, depending upon the location ofthe earth stations. A two-way transmission on a half duplex link toeffect a dialog between two remote points then requires approximately540 ms of transmission time. Effective interactive systems fortransmitting only data should not have a response time greater than anaverage of two seconds.

To provide a full duplex data link between two remote stations utilizingeither point-to-point microwave data links or satellite data links for avideo conferencing network would require that the Network Master resideat one location or the other. This would require all control messagesbetween the two Mixes 221 and 228 to traverse the data link 236.However, if the Network Master resides in each of the Mixes 221 and 228connected to either side of the long haul data link 236, the data linkwould still be maintained but without the versatility provided by asingle Network Master controlling the entire network in a localizednetwork.

Intelligent Interface Module

Referring now to FIG. 11, there is illustrated a schematic block diagramof the interconnection of the audio/video modems and the data linkinterface with the Mixes, as illustrated in FIGS. 9 and 10. A basebandinterface module 238 is provided which is operable to replace one of theSCM circuits 68 illustrated in FIG. 4 and to occupy one of the ports ofthe associated Mix at baseband. The baseband interface 238 receives theincoming and outgoing audio, the incoming and outgoing video and thetransmitted and received data generated internal to the Mix at baseband.The baseband interface 238 has an internal CPU 240 and associatedcircuitry for allowing the baseband interface 238 to operate as anindependent Slave module. The output of the baseband interface 238 iscomprised of baseband audio, video and baseband data. The baseband datacan either be output in an RS-232 format or an RS-422 format, dependingupon the desired output data format. In one configuration, the basebandaudio is input to an audio modem 242 on audio lines 244 to providetwo-way transmission of audio. The video is output to a video modem 246onto video lines 248 to provide two directions of video transmission.The data is output on data lines 250 to both the audio and the videomodems. Alternately, the audio lines 244, the video lines 248 and thedata lines 250 can be input to a data link interface 252 for interfacewith a long haul data link such as a satellite transceiver 254.

In operation, the baseband interface 238 is operable to interface withthe audio and video multiplex circuits in the Mix and also communicatewith the CPU through the RS-232 data path therein. In the Slave mode,the Mix polls the various ports to determine what type of unit isattached thereto. If the baseband interface 238 is attached to one ofthe ports on the Mix in place of one of the SCM networks 68, thisinformation is stored in the internal tables in the Slave and thisinformation transferred to the Network Master for storage in its systemtables. Therefore, when a data link is to be made over the broadbandsystem through the audio and video modems 242 and 246, respectively,information is transmitted to the Mix to indicate channels on thebroadband network 210 over which the data link is to be made. Thisinformation is transmitted from the baseband interface 238 to the audioand video modems 242 and 246 to select a proper channel. In thepreferred embodiment, the audio and video are transmitted over the samechannel. However, they could occupy separate channels, depending uponthe application. Therefore, the baseband interface 238 is operable tointerface between the Mix and either the long haul data link or thebroadband data network to select a data link. The information regardingthe data link and the type of data link is then transmitted back to theNetwork Master for storage in the internal tables thereof.

The audio and video modems 242 and 246, respectively, provide a videoand audio interface to and from a standard broadband network such as aCATV cable. The modem consists of two sections, a transmitter forsourcing video and audio and the receiver for incoming video and audio,both sharing a common package. The video modems use standard broadcastvideo modulation and sound subcarriers to permit the reception ofstandard TV broadcasts and to allow the transmissions from the videomodem to be monitored by standard TV receivers (cable ready). The modemsconform to standard broadcast channel bandwidths, but make no provisionfor encoding or encryption of video or audio. The modems are compatiblewith standard CATV components and can be controlled by the NetworkMaster. They impose no length restrictions on the path, the maximumlength being determined by the broadband system itself. The video modemsprovide for AGC to normalize receive signal levels.

In the preferred embodiment, the LAN utilized is referred to as anAttached Resource Computer local area network (ARC) which ismanufactured by Datapoint Corporation. The LAN modem for interfacingbetween either the Central processor or one of the Mixes is compatiblewith standard CATV components and can be used with single cable systemsor with dual cable systems. The data is transmitted and received using 6mHz wide (standard TV channel space) channels. The transmitted andreceived channels are independently selected using installer configuredswitches (not software programmable). The LAN modem connects through aninterface logic card which provides for automatic echo blanking and datacoding. The transmitter and receiver are implemented as two independentfunctions but share a common package. Delays in the data path of thetransmitter, receiver and interface logic circuits subtract directlyfrom the maximum path length. An additional delay in the data is imposedby the lower velocity factor of most 75 ohm CATV cable.

Mate Processor

Referring now to FIG. 12, there is illustrated a schematic block diagramof the processor portion of the Mate for processing the audio and thevideo. The audio/video processor includes a microprocessor 256 whichinterfaces with the rest of the circuit over an address bus 258 and adata bus 260. An oscillator 262 is provided for generating timingsignals for both microprocessor 256 and the remainder of the circuit.Memory 264 is provided and interfaced with the address bus 258 and is inthe form of both volatile and nonvolatile memory. The volatile memory iscomprised of a Random Access Memory (RAM) and the nonvolatile memory iscomprised of Read Only Memory (ROM). A Serial Interrupt Timer Controller266 is provided for controlling some of the timing tasks of the systemand a dual univeral asynchronous receiver/transmitter (DUART) 268 isalso provided that is interfaced with the address bus. The DUART 268 isoperable to interface with both the Rx/Tx data for the local processorand the Rx/Tx data from the SCM. This provides an input/output (I/O)capability while at the same time handling interrupts.

The UART 268 is interfaced with an analog/digital converter (A/D)circuit 270 which is connected to a volume control that is external tothe cabinet of the Mate. The output of the A/D converter 270 is input tothe UART 268 on a data bus 274. A four digit calling number can be inputon a binary switch 276 for output to the bus 274 to provide the ID forthe Mate.

The data from the microprocessor 256 is input to a data latch 278 foroutput to a latched data bus 280. This data is input to a soundgenerator 282 and a dual digital-to-analog (D/A) converter circuit 284.The sound generator 282 outputs a sound to a triple four-to-one audioswitch 286. The control input of the audio switch 286 is input to thelatched data bus 280 to receive data therefrom. Additionally, the audioswitch 286 receives an audio input from the SCM on a line 288 and anaudio input from the auxiliary input on a line 290. A fourth input isreceived from the microphone or the audio output from the SSIB 18through a voltage controlled amplifier (VCA) 292. The audio from themicrophone is processed through a preamp 294 and then input to a summingcircuit 296 through an enable switch 298. The audio from the SSIBcircuit is processed through a preamp 300 and input to the summingcircuit 296 through an enable switch 302. The output of the summingcircuit 296 is input to the VCA 292. The enable switches 298 and 302 arecontrolled by a control latch 304 which is controlled by signals fromthe data bus 274. The output of the audio switch 286 is connected toeither the SCM or to the auxiliary output.

The audio output of the summing circuit 296 is processed through abandpass filter 306 and input to an absolute value amplifier 308. Theoutput of the absolute value amplifier 308 is input to one input of acomparator 310, the other input of which is connected to the output of apeak detector 312 for comparison therewith. The peak detector 312 isoperable to detect audio levels received from the audio input to thespeaker, and is connected to the output of an audio power amplifier 314which drives the speaker. The input of the audio power amplifier 314 isconnected to the output of a VCA 316, the input of which is connected toa third output of the audio switch 286. This output represents eitherthe audio from the SCM or from the auxiliary input. Both the VCA 292 andthe VCA 316 have the control inputs thereof attached to the D/Aconverter circuit 284. The levels therefore are software controlled bythe microprocessor 256.

A triple three-to-one video switch 318 processes NTSC video from threeinputs. One input is directly from the SCM, one input is directly fromthe camera and processed through an AGC and D/C restoration circuit 320and the other input is received from the auxiliary input afterprocessing by an AGC and D/C restoration circuit 322. The switch 318outputs one output to the SCM, one output to the viewfinder and oneoutput to the auxiliary output or the NTSC/RGB decoder.

A triple two-to-one wideband video switch 324 is provided for receivingone input from an NTSC/RGB decoder and one input from the localprocessor to output a single RGB signal for input to the monitor. A dualtwo-to-one digital switch 326 is provided having one input thereofconnected to the vertical sync and horizontal sync from the NTSC/RBGdecoder and the other input thereof connected to the vertical sync andhorizontal sync from the local processor. The output of the digitalswitch provides the horizontal sync and vertical sync for the monitor.

Voice Operated Gain Switching

Referring now to FIG. 13, there is illustrated a schematic block diagramof the audio paths for a simple two-way video conference. Theoriginating Mate has a microphone 328 which is input to a VCA 330 andthen to the SCM circuit 332. The output of the SCM 332 is then modulatedto the appropriate spectral portion in an SCM cable 334, the path beingrepresented by a directional line 336. This signal is received by an SCM338 in the destination Mate and output to a VCA 340. The output of theVCA 340 is connected to the speaker 342. The destination Mate has amicrophone 344 which is input to the SCM 332 through a VCA 346 and thentransmitted to the SCM 334 along a path represented by a directionalline 348. The SCM 332 processes this received audio and inputs it to aspeaker 350 through a VCA 352. Since, as described above, there is nocross coupling in either the originating Mate or the destination Mate,the cross coupling must occur between the speaker and the microphone ateither the origination Mate or the destination Mate. One feedback pathis illustrated by an arrow 354 between the speaker 342 and themicrophone 344 in the destination Mate and a second feedback path isillustrated by an arrow 356 between the microphone 328 and the speaker350 in the originating Mate.

In operation, a signal is input to the microphone 328 of the originatingMate and passes through the VCA's 330 and 340 and through the SCM cable334 for output from the speaker 342. This leg of the transmission has adefined gain which is adjustable by the VCA's 330 and 340. In a similarmanner, a signal input to the microphone 344 for output from the speaker350 has a gain determined by both the loss in the SCM cable 334 and thegain of the VCA's 346 and 352. One variable factor in the feedback isthe amount of actual feedback between the speaker and microphone.Depending upon the design of the console in either the originating orthe destination Mate and also the surrounding environment, such asreflective surfaces at or near the microphone and speaker, this feedbackpath will vary. If the loop gain increases to greater than one, thesystem will become unstable and oscillate, thereby providing positiveaudio feedback which can be quite irritating in a conference.

To reduce or eliminate the occurrence of positive feedback, it isnecessary to reduce the loop gain to less than unity. This can beaccomplished in number of ways. First, the speaker can always be mutedthe user is talking and the microphone disabled when user is listening.However, this would prevent the user from hearing comments during hisconversation and would effectively become a half duplex audio systemrather than a full duplex system. The second method, and the preferredmethod, is to reduce the speaker gain during speaking by the user andreduce the microphone gain during listening. This effectively reducesthe gain in one leg of the audio path and is accomplished by detectingthe speech or the lack thereof at the microphone and adjusting the gainin accordance with a predetermine gain value.

Referring now to FIG. 14, there is illustrated an alternateinterconnection for a video conference wherein four Mates are involvedin the conference. The originating Mate is illustrated as an SCM 358with a microphone 360 and a speaker 362. The SCM is interconnected witha summing circuit 364 through an SCM cable 366. The summing circuit 364is a part of the Mix. The remaining Mates in the conference arerepresented by SCMs 368, 370 and 374. The SCMs 368-370 and 374 areinterfaced with the summing circuit 364 through SCM cables 376, 378 and380, respectively. The SCMs 368, 370 and 374 have speakers 382, 384 and386, respectively, associated therewith and microphones 388, 390 and392, respectively, associated therewith.

In a multiway conference, an additional feedback path is provided foreach Mate added to the conference. Since these audio paths are inparallel, they will reduce the effective feedback impedance in theoverall audio path, thereby necessitating a slight compensation in theloop gain. Each time an additional Mate is added to a conference, all ofthe Mates in the conference are sent a message from the Network Masteras to the number of Mates in the conference. The amount of gainreduction in either the speaker path or the microphone path is thendecreased accordingly. This is primarily a software function.

Referring now to FIGS. 15a and 15b, there are illustrated graphs ofspeaker volume setting versus microphone and speaker gain for the audiocondition where the user is the speaker and the audio condition wherethe user is the listener. When the user is speaking, his voice isdetected by a voice operated switch (VOX) and outputs a gain controlsignal when the microphone input is above a predetermined threshold.

In FIG. 15a, there is illustrated a graph for microphone gain andspeaker gain as a function of the volume setting of the speaker for thecondition where the VOX is on. The microphone level is illustrated bycurve 394 and the speaker output is denoted by a curve 396. With VOX on,the microphone gain is maintained at a 0 db level. However, as thevolume control for the speaker increases, a gain threshold point 398 isreached and the speaker gain is maintained constant. The threshold point398 is approximately -15 db. As described above, this threshold pointvaries as a number of persons in a video conference changes.

Referring now to FIG. 15b, there is illustrated a graph of volumesetting versus microphone and speaker gain for the condition where VOXis off. The microphone gain is illustrated by a curve 400 and thespeaker gain is illustrated by a curve 402. When the speaker gainreaches the speaker gain threshold point 398 as a result of increasingthe volume by the control on the front panel of the Mate, the microphonegain is decreased by the amount the speaker gain has exceeded thespeaker gain threshold. Therefore, a predetermined amount of attenuationis maintained between the speaker and the microphone to ensure anadequate amount of loss in the audio path to prevent feedback andmaintain the loop gain less than unity.

Mix Processor

Referring now to FIG. 16, there is illustrated a schematic block diagramof the Mix without the SCM circuits, the LAN interface circuit or theRS-232 interface circuits attached. The central processing task isperformed by a microprocessor 404 which outputs addresses to an addressbus 406 and data to a data bus 408. The address and data buses are bothinput to a memory circuit 410 which is comprised of RAM and ROM toprovide additional memory when interfaced with the microprocessor 404. Aserial timer/interrupt controller circuit 412 is also interfaced withthe address and data buses 406 and 408 to provide timing and interruptcontrols for the Mix. An oscillator 414 provides timing for themicroprocessor 404 and the remaining circuit.

Dual UARTS (DUARTS) 416, 418, 420 and 422 are provided that areinterfaced to the address bus and the data bus with each half of each ofthe DUARTS 416-422 being output to two wires of a sixteen wire Rx/Tx bus424 carrying serial data to the SCMs. The DUART 416 has an addressswitch 426 provided for inputting a seven bit address thereto whichdetermines the specific ID of a given Mix. The DUART 418 has an addressswitch 428 connected thereto to provide a seven bit address to determinethe configuration of the Mix. This address determines the Master IDnumber which, as described above, determines the priority of the Mixesduring reconfiguration of the Mix from the Slave mode to the Mastermode. The DUART's 420 and 422 have address switches 430 and 432connected thereto, respectively, for providing additional external datainputs. These are normally used for testing configurations and are notutilized during network operation.

An octal eight-to-one summing switch 433 receives four audio inputs onan eight wire bus 434 and provides eight audio outputs on an eight wirebus 436. One wire from each of the buses 434 and 436 is connected toeach of the SCMs or ports of the Mix. An interface circuit 438 controlsthe summing switch 433 in response to data received from the data bus408. An octal video eight-to-one switch 440 has eight video input lines442 and eight video output lines 444. One of the output lines 444 andone of the input lines 442 are connected to each SCM or port of the Mix.Each of the eight switches in the video switch circuit 440 is controlledby a three-bit word. These three-bit words are output from a parallelI/O output port from each of the DUARTS 416-422. These three bit wordsare generated in response to an address input to the DUARTS 416-422 fromthe address bus 406.

The audio input and output wires 434 and 436, the video input and outputwires 442 and 444 and the data Rx/Tx lines are connected to the eightports through port connectors 446. As described above, each of the portconnectors has associated therewith an audio input and an audio output,a video input and a video output and an Rx data line and a Tx data line.Each of the DUARTS 416-420 has associated therewith indicator lightemitting diodes (LEDs) 448, 450, 452 and 454, respectively. Theseindicator lights are utilized for indicating status and areindependently controlled by microprocessor 404 for various functions.

A parallel interface circuit 456 is provided that interfaces with theaddress bus 406 and the data bus 408 and provides output lines 458 thatare input to the LAN interface connector. This connector interfaces withthe interface circuit for the LAN. A buffer circuit 460 is connected toa serial Rx/Tx output line from the serial timer interrupt controllerwhich has an internal UART. The output of the buffer is connected to aserial connector 462 which interfaces with the RS-232 interface circuit.This RS-232 interface circuit is utilized to drive a dumb terminal forproviding a local indication of the status of each Mix. No controlfunctions are provided through this port.

Station Interface Box

Referring now to FIG. 17, there is illustrated a schematic block diagramof the SSIB 18. The SSIB 18 receives on the input thereof transmittedaudio, received audio, transmitted data, received data and twoconnections for power and ground. The audio is input to one side of ahybrid 464, the other side of which is output from the SSIB 18 forconnection to a conventional telephone handset. The data inputs areconnected to a CPU 466 which provides a local processing capability forthe SSIB 18. The application software is contained in a memory circuit468 that is interfaced with the CPU 466. Additionally, a key pad 470 isprovided in the SSIB 18 to allow the user to input key strokes. The keypad 470 is equivalent to that found on a conventional touchtonetelephone. Interface control circuits 472 are also provided to allow theSSIB 18 to interface with other telephones such as a secretary'stelephone or the main telephone in the office. A Dual ToneMultifrequency (DTMF) decoder 473 is provided for receiving DTMF signalsfrom the handset and converting them to data for transmittal to the Mate10 on the data path. This allows the user to input data through the keysof the handset.

The CPU 466 is operable to interface with the Mate 10 in FIG. 1 througha serial data link provided by the transmitted data line and thereceived data line. This is essentially done in an RS-232 format. Byutilizing the serial data link, only two wires are required forinterfacing all the functions of the SSIB 18 with the Mate 10. Theserial data path also allows the SSIB 18 to operate asynchronous withrespect to the Mate 10 and provides key strokes to the Mate.

The SSIB 18 has the capability to activate either a main buzzer or asecretary buzzer which is at the secretary's station which may beremotely disposed from the Mate 10 itself. When a video call comes in,the Mate sends a "buzz secretary" command to the SSIB and then the SSIBbuzzes the secretary station set for two seconds. The Mate will repeatthe command every four seconds. If after twenty seconds, the secretaryset is not answered, then the Mate will begin chiming locally throughthe speaker of the Mate. If the secretary's phone goes off hook, theMate then answers the call in audio only. After some talk, the secretarypresses the "*" key to buzz the station SSIB. The SSIB then sends achime command to the Mate. This causes the Mate to generate a buzz soundin the Mate to differentiate it from the incoming call chime. Thehandset attached to the SSIB proximate the Mate is then picked up andthe user can talk to the person on the secretary's phone. When thesecretary hangs up, the call is answered in full video.

Local Processor Interface

Referring now to FIG. 18, there is illustrated a schematic block diagramof one form of the interface circuit 44 of FIG. 1 for interfacingbetween the Mate 10 and the local processor 42. The example illustratedin FIG. 18 is for the format utilized by International Business MachinesCorporation for their personal computer referred to as the "IBM PC". TheIBM PC outputs red, green and blue levels and an intensity signal, andit also outputs the horizontal sync and vertical sync signals. Thecircuit of FIG. 18 is operable to translate the various video levels tobe compatible with the RGB levels of the Mate 10 and also to detect thepresence of a brown color signal output by the IBM PC. This brown colorsignal is a peculiarity of the IBM PC which, when not compensated for,results in a different color.

The red input level from the IBM PC is input to a buffer 474, the outputof which is input to another buffer 476. The output of the buffer 476 isconnected to the anode of a diode 480. The cathode of the diode 480 isconnected to one end of a resistor 482, the other end of which isconnected to a node 484. The green signal level from the IBM PC is inputto a buffer 486, the output of which is input to a second buffer 488,the output of which is connected to the anode of a diode 490. Thecathode of the diode 490 is connected to one side of a resistor 492, theother side of which is connected to a node 494. The blue level from theIBM PC is connected to the input of a buffer 496, the output of which isconnected to the input of a buffer 498, the output of which is connectedto the anode of a diode 500. The cathode of the diode 500 is connectedto one end of a resistor 502, the other end of which is connected to anode 504.

The intensity level output by the IBM PC is input to a buffer 506, theoutput of which is connected to the anodes of diodes 508, 510 and 512.The cathode of the diode 508 is connected to the node 504 through aresistor 514, the cathode of the diode 510 is connected to the node 494through a resistor 516 and the cathode of the diode 512 is connected tothe node 484 through a resistor 518. The series diode and resistorconfiguration attached to the output of each of the buffers 476, 488,498 and 506 is operable to provide a level translation of the voltagecompatible with the RGB format of the monitor in the Mate 10. Theresistor values are selected in accordance with the level translationdesired.

For the IBM PC, the output of each of the buffers 474, 486 and 496 isinput to the digital input of a three-input decoder 520. The enableinput is connected to the output of the buffer 506 for enabling theintensity signal. The decoder 520 is utilized to detect the presence ofthe levels of the red, green and blue signals output by the IBM PC forthe color brown. This, as described above, is a peculiarity of the IBMPC and the level must be adjusted slightly for this particular color.The appropriate output from the decoder 520 is selected and inputthrough a diode 522 and a series resistor 524 and connected to the node484 to provide the appropriate level for brown. The output of thedecoder 520 that is selected depends upon the state of the red, greenand blue signals output by the IBM PC for the brown color. This can befound in the application manual on the IBM PC.

The node 484 with the translated red values is input to the base of atransistor 526, the translated green value on the node 494 is input tothe base of a transistor 528 and the translated blue value on the node504 is input to the base of a transistor 530. The collectors of thetransistors 526-530 are connected to a positive voltage supply and theemitters thereof are connected to a minus voltage supply throughresistors 532, 534 and 536, respectively. The emitter of the transistor526 is connected to the red video input on the Mate 10 through a seriesresistor 538. The emitter of the transistor 528 is input to the greenvideo input of the Mate 10 through a series resistor 540. The emitter ofthe transistor 530 is input to the blue video input of the Mate 10through a series resistor 542.

The horizontal sync signal output by the IBM PC is input to a buffer544, the output of which is input to the clock input of a one shotcircuit 546. The output of the one shot circuit 546 is input to theclock input of a retriggerable one shot circuit 548. The output of theone shot 548 drives the base of an NPN transistor 550 which is connectedas an emitter follower, the output of which provides the invertedhorizontal sync signal. The one shot circuit 546 has the timing thereofcontrolled by a variable resistor 552 and a capacitor 554 for varyingthe duration of the one shot 548. The retriggerable one shot 548 iscontrolled by a resistor 556 and a capacitor 558. The one shots 546 and548 are of the type 74123 manufactured by Texas InstrumentsIncorporated. They provide a horizontal phase control to adjust thephase between the horizontal sync of the IBM PC and the horizontal syncof the monitor.

The vertical sync output by the IBM PC is input to an inverter 560, theoutput of which drives the base of an NPN transistor 562. The NPNtransistor 562 is connected in an emitter follower configuration toprovide the inverted vertical sync input to the monitor.

The IBM PC also outputs transmitted and received data in an RS-232format, which is compatible with the format of the Mate. The Tx outputof the IBM PC is input to the input of three series inverter circuits564, 566 and 568 through a series resistor 570. The input of theinverter 564 is connected to a minus voltage supply through a resistor572. A transient suppressor 574 is connected between the input of theinverter 564 and ground. The output of the third inverter 568 in theseries configuration provides the Rx output to the monitor through aseries resistor 576. A capacitor 578 is connected between the Rx outputand ground. In the opposite direction, the Tx output of the Mate isconnected to the input of three series configured inverters 580, 582 and584 through a series resistor 586. A capacitor 588 is connected betweenground and the input of the first inverter 580 in the seriesconfiguration. The output inverter 584 in the series configurationprovides the Rx input to the IBM PC through a series resistor 590. Atransient suppressor 592 is connected between the output of the thirdinverter 584 in the series configuration and ground. The inverters564-568 and 580-584 are of the type 7404 manufactured by TexasInstrument Incorporated. The buffers 474-476, 486-488, 496-498, 506 and544 are all of the type HCT541 manufactured by National Semiconductor.Although the circuit of FIG. 18 is utilized for interfacing between theIBM PC and the Mate 10, it should be understood that the circuit iscustomized for each computer or local processor that is interfaced withthe monitor 12 such that a predetermined RGB level, horizontal andvertical sync levels and serial data interface is provided.

Mate Processor Schematic

Referring now to FIG. 19, there is illustrated a schematic diagram ofthe processing portion of the Mate 10. A microprocessor 594 is utilizedto provide a sixteen bit address output and an eight bit data output.The processor is a Z80A processor manufactured by Mostek Corp. Theaddress outputs of the microprocessor 594 are connected to a sixteen bitwide address bus 596. The data outputs are connected to an eight bitwide data bus 598. The first eight bits of the address bus 596 are inputto a quad D-type latch 600, the output of which is connected to thefirst eight bits of a sixteen bit latched address bus 602. The othereight bits of the address bus 596 are input to the D-input of an octalD-type latch 604, the outputs of which are connected to the remainingeight bits of the latched address bus 602.

A 32K×8 Erasable Programmable Read Only Memory (EPROM) 606 has theaddress inputs A0-A13 connected to the first fourteen bits of thelatched address bus 602 and the data outputs on an eight bit wide dataoutput connected to the data bus 598. An 8K×8 Random Access Memory (RAM)608 has the address inputs A0-A10 and A12 connected to the respectiveaddress lines on the latched address bus 602 and the data output portson an eight bit wide data output connected to the data bus 598.

A bus and timing control circuit 610 is provided to generate varioustiming controls such as chip select resets and miscellaneous timingsignals. An oscillator 612 generates timing signals which are input tothe clock input of a J-K flip-flop 614. The Q-output of the flip-flop614 is connected through an inverter 616 to a node 618. The invertedQ-output is connected through an inverter 620 to a node 622. The node618 provides the clock input to the microprocessor 594 and also timingreference signals to the bus controller 610.

An interrupt timer circuit 624 is provided for generating timing andinterrupt signals for the microprocessor 594. The interrupt timer 624has seven data inputs connected to the data bus 598. The interrupt timer624 also has an internal UART and a seven bit wide latched output forlatching data thereto. The timer 624 has four address inputs A0-A3 whichare connected to the first four bits of the address bus 596. A resetinput is received from the inverted form of a power reset input (POR).The interrupt request signal is output from the interrupt timer 624 forconnection to the interrupt input of the microprocessor 594. The read(RD) and write (WR) inputs of the interrupt timer 624 are also connectedto the microprocessor 594 with the clock input being connected to thenode 622 for receiving the inverted clock from out of the flip-flop 614.The internal UART of the interrupt timer 624 is utilized to process theserial data from the SSIB 18. The serial input is connected to an Rx(SSIB) line 626 and a serial output port is connected to a line 627labeled Tx (SSIB).

The interrupt timer 624 outputs eight parallel input/output bits whichare selectable under the control of the microprocessor 594. These arelabeled P0-P7. The outputs P0 and P1 are connected to LEDs 628 and 630for providing status signals. The LEDs 628 and 630 are connected to apositive voltage through pull up resistors. The input P2 is connected toa switch 632 for selecting between soft and loud on the audio. When theinput P2 is grounded, this indicates an active state or a soft signal.The other state of the switch represents the loud state. The switch 632is on the rear panel of the Mate for user access. The port P4 isconnected to the output of a debounce circuit 634 which is connected totwo lines 636, which are connected to the mode switch on the front panelof Mate 10. Port P5 supplies a READY signal for an A/D converter andport P6 receives the inverted privacy signal from the privacy switch onthe front panel which is inverted through an inverter circuit 638. Theprivacy switch allows the user to disconnect his microphone and camerafrom the SCM. Port P7 is connected to a VOX ON signal which indicatesthat voice has been detected for a predetermined duration of time. Theinterrupt timer 624 is of the type manufactured by Mostek Corporation,part No. MK3801-4.

A DUART 640 is provided for receiving the serial data from the SCM andthe local processor through the interface circuit 44. The DUART 640 hasan eight bit data input that is interfaced with the data bus 598 andfour address inputs A0-A3 which are connected to the first four bits ofthe address bus 596. The DUART 640 has seven parallel output portsOP0-OP6 for latching output data from the data bus 598. Additionally,the DUART 640 has seven input ports IP0-IP6 for receiving data andlatching it to the data bus 598. A seven switch dual inline package(DIP) switch 642 is interfaced with the input ports IP0-IP6. The resetinput is connected to the POR signal and the read (RD) and write (WR)inputs are connected to the microprocessor 594. The X1 input isconnected to the node 622 to receive the inverted clock signal output bythe flip-flop 614.

The SCM Rx signal is input on the RxA port and the SCM Tx signal isoutput from the TxA port. In a similar manner, the local processor Rxsignal is input on the RxB port and the Tx output for the localprocessor is output from the TxB port. The interrupt input of the DUART640 is connected to the port P3 from the interrupt timer 624.

The output ports OP0-OP5 provide Select outputs to the video switch. Theports OP0 and OP1 provide monitor select outputs for selecting themonitor, the ports OP2 and OP3 provide video selects for selectinginternal video and the ports OP4 and OP5 provide SCM selects forselecting video from the SCM output which is network video. The port OP6is connected to an LED 644 for indicating status which is pulled downfrom a positive voltage supply through a series resistor. The port OP7is connected to a signal DACSEL for selecting the digital to analogconverter.

The first three bits of the latched address bus 602 are input to an I/Odecoder 646 which is a three-to-eight decoder of the type 74138manufactured by Texas Instruments Incorporated. The three bit input fromthe latched address inputs LA0-LA2 selects one of eight outputs. Thefirst output is an inverted DACWR signal for writing to thedigital-to-analog converter, the second output is the inverted AUDSTR0signal for operation with the audio circuit, the third output is theinverted AUDSTR1 output for selecting one of the audio multiplexers. Thesecond and third outputs are clock signals for latching data in datalatches 648 and 650, respectively. The fifth and sixth outputs are inputto a steering circuit 652 to provide the signals BDIR and BCI which areutilized for the sound generator or circuit.

A two-to-four decoder 654 is provided having the two address inputsconnected to the address lines LA0 and LA1 from the latched address bus602 with the four outputs thereof providing Read and Write controlsignals for analog-to-digital conversion. An octal D-type latch circuit656 is provided for latching data from the data bus 598 to provide theWRD0-WRD7 signals which are the control signals for the audio switch.The latched output of the latch 656 is also input to the D-type latch650 which is controlled by the fourth output of the I/O decoder 646. Theoutputs of the latch 650 provide various synchronizing signals.

The telephone number or the ID of a specific Mate is input on fourrotary switches 660, 662, 664 and 666. Each of the switches 660-666 isoperable to provide binary output data. The binary outputs of the twoswitches 664 and 666 are input to the inputs of an octal line driver668. The binary outputs of the two switches 660 and 662 are input to anoctal line driver 670. The line drivers 668 and 670 are of the type74LS244 manufactured by Texas Instruments Incorporated. The outputs ofboth of the line drivers 668 and 670 are input to the data bus 598 andare individually selectable. The switches 660 and 662 comprise the highportion of the telephone number and the switches 664 and 666 comprisethe low portion of the telephone number. The inverted form of the signalHIDRD is input to the enable inputs of the line driver 670 and theinverted form of a signal LIDRD is inverted to the enable inputs of theline driver 668. These signals are generated from the two-to-fourdecoder 654.

An analog-to-digital converter 672 is provided which has an eight bitdata output port with each of the bits connected to the correspondingbits in the data bus 598. The interrupt input is connected to theinverted form of the signal A/D READY and the clock input is connectedto the A/D CLK signal output by the interrupt timer 624. The READ andWRITE signals are connected to two outputs of the two-to-four decoder654 to provide timing signals therefor. The analog input is connected tothe voltage sense output of the voltage control knob. This isessentially the wiper on a potentiometer and constitutes the volumecontrol. As the voltage varies, this voltage is converted to a digitalvalue and read by the microprocessor 594.

A sound generator circuit 674 is provided which has an eight bit datainput port connected to the data bus 598. The sound generator circuit674 outputs three signals which are each input to bandpass filters 676,678 and 680. The outputs of the bandpass filters 676-680 are input to asumming circuit 683 to provide the synthesized sound signal SDGN. Thereset input of the sound generator 674 is connected to the inverted formof the POR signal and the clock input is connected to a timing signaloutput by the interrupt timer circuit 624. The steering control BDIR andBCI output by the steering circuit 652 also input to the sound generatorcircuit.

The audio switch is comprised of three quad analog switches 682, 684 and686. The switches 682, 684 and 686 receive the audio inputs CNTRLMICfrom the microphone, the SCM audio signal AUDWO, the auxiliary audiosignal AUXAD and the signal generator signal SDGEN. Each of thesesignals is individually controlled for output therefrom through summingresistors 688, 690, 692 and 694 to a node 696. The switches arecontrolled by the signals output by the data latch 656 with the WRD4signal controlling the SDGN signal, the WRD5 signal controlling theAUXAUD signal, the WRD6 signal controlling the AUDWO signal and the WRD7signal controlling the CNTRLMIC signal. The summed voltage on the node696 is input to an op amp 698 which is configured as a summingamplifier. The output constitutes the audio output for the SCM.

The analog switch gates 684 and 686 are controlled by the signalsWRD0-WRD3. WRD3 controls the selection of the CNTRLMIC signal, WRD2controls the selection of the AUDWO signal, WRD1 controls the selectionof the AUX AUD signal and WRD0 controls the selection of the SDGNsignal. The four outputs of the analog switch gate 684 are summed with asumming op amp 700 through four resistors 702, 704, 706 and 708. Theoutput of the summing op amp 700 comprises the auxiliary output. Thefour outputs of the analog switch gate 686 are summed with an op amp 710through four series resistors 712, 714, 716 and 718. The output of theop amp 710 comprises the audio for this speaker.

The video switch is comprised of three three-to-one video switches 720,722 and 724 which are of the type HI-524 manufactured by HarrisSemiconductors. Each of the switches 720-724 receives the video inputfrom the SCM, the video input from the camera and the video input fromthe auxiliary video port. Each of these videos has been processedthrough an AGC circuit (not shown). The switch 720 is operable to switchvideo to the monitor and auxiliary port, the switch 722 is operable toswitch video to the SCM and the switch 724 is operable to switch videoto the viewfinder. The address inputs A1 and A0 of the switch 720 areconnected to the monitor select signals MONSEL0-MONSEL1 output by theoutput ports of the DUART 640, the address inputs A1 and A0 of theswitch 722 are controlled by the SCMSEL1 and SCMSEL0 signals output bythe DUART 640 and the address inputs A1 and A0 of the switch 724 arecontrolled by the signals VFINSCL1 and VFINSCL0 that are output by theDUART 640.

The output of the switch 720 is input to the base of the PNP transistor726 through a series resistor 728. The collector of the transistor 726is connected to a minus voltage supply and the emitter thereof isconnected to a positive voltage supply through a series resister 730.The emitter of the transistor 726 is also input to the base of NPNtransistors 732 and 734 through series resistors 736 and 738,respectively. The transistors 732 and 734 have the collectors thereofconnected to the positive voltage supply and the emitters thereofconnected to the negative supply through resistors 739 and 741,respectively. The emitter of the transistor 732 comprises the videooutput of the auxiliary port and the emitter of the transistor 734comprises the video output for the NTSC/RGB decoder.

The output of the switch 722 is input to the base of a PNP transistor740 through a series resistor 742. The transistor 740 has the collectorthereof connected to the negative voltage supply and the emitter thereofconnected to the positive voltage supply through a resistor 744. Theemitter of the transistor 740 is also connected to the base of a PNPtransistor 746. The transistor 746 has the collector thereof connectedto the positive voltage supply and the emitter thereof connected to thenegative voltage supply through a resistor 748. The emitter of thetransistor 746 comprises the video output to the SCM. The output of theswitch 724 is input to the base of an NPN transistor 750 through aseries resistor 752. The transistor 750 has the collector thereofconnected to the positive voltage supply and the emitter thereofconnected to the negative voltage supply through a series resistor 754.The emitter of transistor 750 is the video output to the viewfinder.

The output from the microphone is input to the negative input of an opamp 756 through a series configured capacitor 758 and resistor 760. Thepositive input of op amp 756 is connected to ground. A parallelconfigured capacitor 762 and resistor 764 are connected between thenegative input and the output thereof. The op amp 756 is a preamplifier,the output of which is connected to the input of an enable switch 766.The enable switch 766 is controlled by the inverter from the microphoneenable signal MICEN output by the data latch 648. The output of theenable switch 766 is connected to the negative input of an op amp 768through a series configured capacitor 770 and resistor 772. A parallelconfigured capacitor 773 and 774 are connected between the negativeinput of the op amp 768 and the output thereof. The positive inputthereof is connected to ground.

The audio input from the SSIB is connected to the negative input of anop amp 776 through a series figured capacitor 778 and resistor 780. Aparallel configured capacitor 782 and resistor 784 are connected betweenthe negative input of the op amp 776 and the output thereof. Thepositive input thereof is connected to ground. The output of the op amp776 is connected to the input of an enable switch 786 which iscontrolled by the inverted form of the SSIB enable signal SSIBEN whichis output by the data latch 648. The output of the enable switch isinput to the negative input of the op amp 768 through a seriesconfigured capacitor 788 and resistor 790. The op amp 768 thereforefunctions as a summing amplifier and an additional stage of gain in theaudio path. The output of the op amp 768 comprises the microphonepreamplifier output MPROUT.

The microphone preamplifier output MPROUT is input to the input of avoltage controlled amplifier (VCA) 792 through a series configuredcapacitor 794 and resistor 796. The output of the VCA 792 is input tothe negative input of an op amp 798, the positive input of which isconnected to ground. The output of the op amp 798 is connected to a node800 through a series resistor 802. The node 800 comprises the microphoneoutput to the switches 682, 684 and 686 for summing thereof. A parallelconfigured capacitor 804 and resistor 806 are connected between thenegative input of the op amp 798 and the node 800. The VCA 792 is of thetype 2150 manufactured by DBX Corporation.

The VCA 792 is controlled by a digital-to-analog (D/A) converter 808which has the data inputs D0-D7 connected to the data bus 598. Theselect input is the DACSEL signal output by the DUART 640 and the Writeinput is connected to the inverted form of the DACWR signal output bythe decoder 646. The D/A converter 808 is a dual decoder with anA-output port and a B-output port labeled "OUTA" and "OUTB". The "OUTB"port is connected to the negative input of an op amp 810, the positiveinput of which is connected to ground. The output of the op amp 810 isconnected to one of the control inputs of the VCA 792 and also the RFBBoutput of the D/A converter 808. A capacitor 812 is connected betweenthe RFBB output and the OUTB output of the D/A converter 808. Twooppositely directed diodes 814 and 816 are connected in parallel betweenthe output of the op amp 810 and ground to provide a voltage limitingfunction.

A second VCA 818 is also provided which is controlled by the D/Aconverter 808 and has the input thereof connected to the output of theanalog gate 686 which provides the speaker audio and the signal ispassed through a series configured capacitor 820 and resistor 822. Theoutput OUTA of the D/A converter 808 is connected to the negative inputof an op amp 824, the positive input of which is connected to ground.The output of the op amp 824 is connected to the control input of theVCA 818 and also to the RFBA output of the D/A converter 808. Acapacitor 826 is connected between the OUTA and RFBA outputs. Twooppositely directed diodes 828 and 830 are disposed in parallel betweenthe output of the op amp 824 and ground to provide a voltage limitingcapability.

The output of the VCA 818 is connected to the negative input of an opamp 832, the positive input of which is connected to ground. A parallelconfigured capacitor 834 and resistor 836 are connected between theoutput and the negative input thereof. The output of the op amp 832 isconnected to the input of an enable switch 838 which is controlled bythe mute control signal MUTESPKR for the speaker which is output by thedata latch 648. The output of the enable switch 838 is the correctedspeaker volume level which is input to the positive input of an outputdriver 840 through a capacitor 842. The negative input of the driver 840is connected to a node 844 through a capacitor 846, the node 844 beingconnected to ground through a series resistor 848. A series resistor 850is connected between the node 844 and the output of the driver 840 witha capacitor 852 connected between the output and ground. The output isalso connected to the speaker through a series capacitor 854.

The microphone preamp output MPREOUT is connected to a node 856 througha resistor 858. A capacitor 860 is connected between the negative inputof an op amp 862 and the node 856. The positive input of the op amp 862is connected to ground and the node 856 is also connected to groundthrough a series resistor 864. A capacitor 866 is connected between thenode 856 and the output of the op amp 862, and a resistor 868 isconnected between the negative input of the op amp 862 and the outputthereof. The op amp 862 is configured as a bandpass filter.

The output of the op amp 862 is connected to the negative input of an opamp 880 through a series resistor 882 and also to the negative input ofan op amp 884 through a series resistor 886. The op amp 884 has a diode888 connected between the output and the negative input in a feedbackconfiguration with the cathode thereof connected to the negative input.A diode 890 has the cathode thereof connected to the output of the opamp 880 and the anode thereof connected to a node 892. The node 892 isconnected to the negative input of the op amp 884 through a seriesresistor 894. A series resistor 896 is connected between the node 892and the negative input of the op amp 880. The op amp 884 has a resistor898 connected between the output and the negative input thereof with thepositive input thereof being connected to ground. The op amps 884 and880 and the op amp 862 comprise an absolute value amplifier wherein theoutput of the op amp 884 is proportionally related to the absolute valueof the input voltage. This voltage is compared with the detected outputof the speaker on the driver output to the speaker for comparison ingenerating the VOX.

The output of the driver 840 for driving the speaker is input through apotentiometer 900 to the positive input of an op amp 902 and also to thenegative input of an op amp 904 through a series resistor 906. The opamp 902 has a diode 908 connected in feedback between the negative inputand the output with the cathode thereof connected to the output. A diode910 has the anode thereof connected to the output of the op amp 902 andthe cathode thereof connected to a node 912. The op amp 904 has a diode914 connected between the negative input and the output thereof with thecathode connected to the output. A diode 916 has the anode thereofconnected to the output of the op amp 904 and the cathode thereofconnected to the output node 912. A resistor 918 is connected betweenthe negative input of the op amp 904 and the node 912. A resistor 920 isconnected between the negative input of the op amp 902 and the node 912.The op amps 902 and 904 are configured as a peak detector.

The node 912 is connected to positive input of a comparator 922 througha series resistor 924. The negative input of the comparator is connectedthrough the wiper of a potentiometer 926 to the output of the op amp 884which is the absolute value of the microphone preamp signal. Anadditional series resistor is placed in series with the output of the opamp 884 and the potentiometer 926 with a switch 928 disposedthereacross. The switch 928 is controlled by a switch on the front panelfor noisy and quite environments. When the resistor 924 is taken out ofthe circuit, this is equivalent to changing the threshold for the VOX.The comparator 922 compares the detected output from the speaker withthe detected output from the microphone to output a signal when thedetected output from the speaker is higher than that of the microphone.

The output of the comparator 922 is input to the negative input of an opamp 930 through a series resistor 32, the output of which comprises theVOX ON signal which goes to the interrupt timer circuit 624. Thepositive input of the op amp 930 is connected to the positive voltagesupply through a resistor 933 and also to the output of the op amp 930through a series resistor 934.

Mix Schematic

Referring now to FIGS. 20a to 20b, there is illustrated a schematicdiagram of the processing portion of the Mix. A microprocessor 970 isprovided that is identical to the microprocessor 594 in the Mate 10. Themicroprocessor 970 is of the type Z80A manufactured by MostekCorporation and has a sixteen bit address input attached to an addressbus 972 and an eight bit data port interfaced with a data bus 974. Thefirst eight bits of the address bus 972 are input to a D-type latch 976to interface with the first eight bits of a latched address bus 978. Theremaining eight bits of the address bus 972 are latched to the remainingeight bits of the latched address bus 978 through a D-type latch 980.

An EPROM 982 is provided that has the address inputs interfaced with thelatched address bits LA0-LA12 on the latched address bus 978 and sevendata outputs interfaced with the data bus 974. An ElectricallyProgrammable Read Only Memory (EPROM) 984 and a Random Access Memory(RAM) 986 are provided, each having the address inputs interfaced withLA0-LA12 on the latched address bus 978 and the seven bit data outputsinterfaced with the data bus 974. The chip enable inputs of the EPROM982, the EPROM 984 and the RAM 986 are connected to outputs of athree-to-eight decoder 988, which has the address inputs thereofconnected to the latched address bus 978. The three-to-eight decoder 988is activated by internal timing control signals for selection thereofand decoding of the input latched address bits LA13-LA15.

A reference oscillator 990 is input to the clock input of a J-Kflip-flop 992, the Q-output of which is input through an inverter 994 toprovide a clock signal SCLK and the inverted Q-output of which is passedthrough inverter 996 to provide the clock signal for the microprocessor970. The SCLK provides timing signals for the remainder of theprocessing section of the Mix. An interrupt controller 998 is providedwhich is similar to the interrupt timer controller 624 on the Mate 10.The data ports of the interrupt timer controller 998 are interfaced withthe data bus 974 and the address inputs are interfaced with the addresslines A0-A3 on the address bus 972. The serial input (SO) of theinterrupt timer 998 is connected to the output of an RS-232 receivercircuit 1000 and the serial output of the interrupt controller timer 998is connected to the input of an RS-232 transmitter circuit 1002. Theserial input data of the RS-232 receiver circuit 1000 is provided fromthe auxiliary input port, and the transmit output of the RS-232 transmitcircuit is connected to the auxiliary transmit port. The interrupt timercontroller provides various interrupt requests and other controlssimilar to the interrupt timer 624 in the Mate and is interfaced with abus timing control circuit 1004, which provides various control signalsfor bus timing. These are signals such as chip selects, etc. for thevarious chips in the circuit.

The Mix 10 has eight ports for receiving audio, data and video. Thedemodulated serial data is received and processed by UARTS. In thepreferred embodiment, dual UARTS (DUART) are utilized. For simplicitypurposes, only one DUART 1006 is illustrated for receiving serial dataR×2 from port two and R×3 from port three and transmitting serial dataT×2 to port two and T×3 to port three. The data inputs D0-D7 areinterfaced with the data bus 974 and the address inputs are interfacedwith the address bits A0-A3 on the address bus 972.

The DUART 1006 has parallel output ports OP0-OP7 and parallel inputports IP0-IP6. The parallel output ports OP0-OP2 are operable togenerate a three bit select word for selecting one of eight video linescorresponding to port two and the parallel output lines OP3-OP5 comprisea three bit word for selecting one of eight video lines for connectionto port three. The parallel output lines OP6 and OP7 are connected tothe cathodes of LED's 1008 and 1010, respectively, to provide statussignals. The LED's 1008 and 1010 are connected to a positive supplythrough voltage dropping resistors 1012 and 1014, respectively.

The parallel input lines IP0-IP6 are each connected to ground through aswitch input of the dual-in-line package (DIP) switch 1016. The switch1016 is operable to provide an address such as the Mix ID or theconfiguration of the Mix. These are similar to the switches 426-432 inFIG. 16. The READ and WRITE function of the DUART 1006 is controlled bythe microprocessor 970 and the chip select function is controlled by thebus timing and control circuit 1004. The Reset comes from a Power OnReset signal (POR) generated from a reset circuit (not shown).

The video select line of each of the DUARTS, as described above, is athree-bit word which determines which of eight video ports is selectedfor association with a particular video port. Each of the three-bitvideo select words is input to an eight-to-one analog multiplexer forreceiving the video output from the SCM from each of the eight ports andselecting one of them in conjunction with the signal. For simplicitypurposes, only the eight-to-one multiplexer for port three will bedescribed in detail.

The eight-to-one video switch for port three is comprised of afour-to-one analog switch 1018 and a four-to-one analog switch 1020. Theswitch 1018 receives on the four inputs thereof the signalsVIDIN4-VIDIN7 corresponding to video inputs from ports four to seven.The switch 1020 receives the video input signals VIDIN0-VIDIN3corresponding to video signals from the ports zero through three. Theaddress inputs A0-A1 of the switches 1018 and 1020 are connected to theselect signals V3SEL0 and V3SEL1. The enable input of the switch 1020 isconnected to the select signal V3SEL2. The enable signal of the switch1018 is connected to the inverted form of the select signal V3SEL2 whichis passed through an inverter 1022. The inverted form of a select signalV2SEL2 is also received from an inverter 1024 for use with theeight-to-one switch associated with port two (not shown) The switchesare of the type HI524 manufactured by Hitachi.

The output of the switch 1018 is input to the base of an NPN transistor1026, the collector of which is connected to a positive voltage supplyand the emitter of which is connected to a node 1028. The output of theswitch 1020 is connected to the base of a transistor 1030, the collectorof which is connected to the positive voltage supply and the emitter ofwhich is connected to the node 1028. The node 1028 is connected to theinput of a PNP transistor 1032. The transistors 1026, 1030 and 1032 arevideo transistors that operate at video frequencies.

The transistor 1032 has the base thereof connected to a negative supplythrough a resistor 1034 and the collector thereof connected to thenegative supply. The emitter of transistor 1032 is connected to thepositive input of a video op amp 1036 and also to a positive supplyvoltage through a resistor 1038. The emitter is also connected to groundthrough a resistor 1040. The negative input of the video op amp 1036 isconnected to ground through a resistor 1042 and also to the outputthereof through a resistor 1044. The video op amp 1036 is of the typeHA-5195 manufactured by Harris Semiconductors. The output of the op amp1036 is the video output VIDOUT3 for port three.

In operation, the three-bit select word selects one of the ports on eachof the switches 1018 and 1020 and also enables only one of the switches1018 and 1020. The transistors 1026 and 1030 on the outputs of theswitches 1018 and 1020 are connected in parallel such that the signalappearing on node 1028 is comprised of only one video signal. Therefore,the switches 1018 and 1020 do not have the outputs thereof summed.

The audio switching utilizes a three-to-eight decoder 1045 and an octalD-type latch 1047. The three address inputs of the decoder 1045 areconnected to the address bits LA0-LA2 of the latched address bus 978 andthe eight outputs provide the audio write lines AUDSTR0-AUDSTR7. TheD-type latch 1047 has the D-inputs thereof connected to the data bus 974with the eight outputs thereof constituting the audio select lines. Boththe decoder 1045 and the latch 1047 are enabled by the bus timing andcontrol circuit 1004.

The demodulated audio inputs AUDIN0-AUDIN8 from the SCM are input toeight eight-to-one summing switches such that the audio from all eightports can be summed and output to each of the ports. This is utilized inthe conference mode. This is to be distinguished from the video switchwherein only one video can be selected and transmitted out from the Mix.For simplicity purposes, only the audio switch for port one will bedescribed.

A quad analog gate 1046 has the audio inputs AUDIN0-AUDIN3 input to thefour inputs thereof and the control signals AUDSWD0-AUDSWD3 connected tothe respective control input thereof. A quad analog gate 1048 has theaudio input signals AUDIN4-AUDIN7 connected to the four inputs thereofand the control inputs AUDSWD4-AUDSWD7 input to the control inputthereof. Both of the quad analog gates 1046 and 1048 are selected by theinverted form of the WRITE line AUDSTH1 output by the decoder 1045.

The four outputs of the quad analog gate 1046 are connected to a commonnode 1050 through resistors 1052, 1054, 1056 and 1058, respectively. Thefour outputs of the quad analog gate 1048 are connected to the node 1050through series resistors 1060, 1062, 1064 and 1066, respectively. Thenode 1050 is connected to the negative input of a summing op amp 1068,the positive input of which is connected to ground. A resistor 1070 isconnected between the negative input and the output thereof and acapacitor 1072 is connected in parallel with the resistor 1070. Theoutput of the op amp 1068 comprises the audio output AUDOUT1 for portone.

To interface with the local area network on which the Network Masterresides, it is necessary to provide timing signals for the LAN interfacemodule. The latched address data on the latched address bus 978 is inputto line drivers 1061, 1063 and 1065 to buffer the address bits LA0-LA10for output to the LAN interface card. The output is labeled LA0'-LA10'.In addition, a buffer 1067 is provided to buffer timing signals from thebus timing and control circuit 1004. The data from the data bus 974 isbuffered through a bidirectional transceiver 1069 for output to the LANaddress bus.

Network Messages

During communications on the network, the Network Master controls thevideo and audio data links on the network, maintains an updated table ofnetwork status and also constantly updates the status of the network.Communications between the Network Master, the Mixes and the variousMates is through the 9600 baud serial data link when the Network Masterresides in one of the Mixes. As described above, the serial data is FMmodulated on a subcarrier and then modulated onto a primary carrier of70 MHz or 170 MHz, depending upon the direction of transmission on theSCM line. It provides a two-way data path between Mates and theirassociated Mixes, between Mixes in the Slave mode and the Network Masterand between the Network Master and the Mixes. When the Network Masterresides in the central processor or a LAN, the data link between theNetwork Master and the Mixes is through the LAN.

The messages that are transmitted between various devices on the networkare System messages, Control messages or Data messages. System messagesconcern the validation and synchronization of network communications andare messages such as Link Requests and Responses, Statistics Requestsand Responses, Communication Errors, Notification of Power-On and/orResets and Acknowledgements. Although some System messages requireresponses, as will be described hereinbelow, other System messages donot require an answer from the receiving device and merely perform anotification function. Control messages initiate a communicationsexchange between two devices. They inquire about status, control videointercom calls and adjust the Mates audio and video input and output.Data messages are answers to control messages, usually not requiring aresponse.

When a message is placed onto the network, it is addressed to a specificdevice and coded with the originating device's ID and routed to the porton which the addressed device resides. The messages are comprised of aheader portion which includes a source ID, the destination ID, thelength of data in the message, the system ID and the task ID. A secondportion is dedicated to the message content and a third portion isdedicated to error control and is generally referred to as "verticalredundancy check" (VRC). The source ID identifies the device from wherethe message was originated and the destination ID identifies the devicethat is to receive the message. The length of data portion indicates anumber of bytes remaining in the message up to but not including the VRCportion. The system ID distinguishes all video conferencing messagesfrom all other types. The task ID contains information regarding theport number and the retry bit. The message content is comprised ofapproximately one to twenty-five bytes of data and provides conferencingcontrol and verification of communication. As described above, the VRCportion verifies reliability of data.

The device identification ID is a two-byte number used to determine thesource or destination of a video intercom network message. In themessage, the device ID is represented with the least significant bytefirst. In the preferred embodiment, the Network Master is alwaysprovided a device ID of "00", the Mixes are provided device ID's ofbetween "01" through "255" and the Mates are provided device ID's ofbetween "256" through "9999". The SSIB and the local processor areprovided with the same ID for all the Mates in the system since only onecommunication path is provided therebetween.

The task ID is comprised of one byte of data which is divided into twofields. The first field is four bits wide and it provides the portnumber which is the port used to send and receive messages. In the Mate,a value of "0" indicates communication with either the Mix or theNetwork Master. A value of "1" indicates communication with the localprocessor and a value of "2" indicates communication with the SSIB. Thesecond field is the retry bit. The retry bit is either a value of "0",indicating that it is an original message or at a value of "1",indicating retransmission. This byte is utilized to indicate to thereceiving device that the message is a retry in the event that priordata was received and the only error was that the acknowledgement wasnot received by the originating device. In this instance, the receivingdevice knows that the data is redundant and all that is required is toresend the acknowledgement signal.

Placing Call with Mate

Referring now to FIG. 21, there is illustrated a flowchart for theoperation of the Mate when placing a call over the network. The programis initiated at a START block 1080 and proceeds to a decision block 1082to determine if an incoming call has been received. This is the idlestate of the Mate wherein it waits to see if some form of incomingmessage is received from the data port. Although the message may bereceived from the local processor, this situation is not included in theflowchart of FIG. 21 for simplicity purposes.

There are two messages that can be received. If the message is from theNetwork Master, this is an incoming call and a message is in the form ofa "Request for an Incoming Call" from the Network Master. This isillustrated by the "Y" path from the decision block 1082 to a functionblock 1084 to provide a Ring-Back which is input to the SCM cable 16 fortransmission to the originating Mate. When the Mate receives the Requestfor Incoming Call message, the Mate acknowledges with an Acceptresponse, turns on the camera, turns off the auxiliary audio and thenprovides a single "Ring" output to the user, as indicated by a functionblock 1086. This ring is in the form of a "chime". However, this signalcould be routed through the SSIB 18 to the secretary's phone. Theprogram then flows to a decision block 1088 to determine if the "VideoRing" feature has been selected. If it has not been selected, theprogram flows to a function block 1090 to provide a continuous ringoutput.

If the Video Ring feature has been selected, the program flows to adecision block 1092 to determine if the viewfinder has been selected asthe display for the incoming call. If yes, the program flows along the"Y" patch to a function block 1094 and displays the incoming video onthe viewfinder. However, if the viewfinder has not been selected, theprogram flows along the "N" path to a function block 1096 to display theincoming video on the monitor.

When receiving an incoming call, the first decision that has to be madeby the Network Master is whether a path is available. If the path isavailable, then the Network Master must determine if the Mate is busy.This information is contained in status tables at the location of theNetwork Master. Once it is determined that both a path is available andthe destination Mate is not busy, the audio and video link are made andthe status tables in the Network Master updated regarding the use ofaudio/video paths. The result is that the audio and video from theoriginating Mate, if transmitted, are connected to the actual interfacebetween the destination Mate and the rest of the network. Therefore, thedestination Mate can determine whether to receive the video and/or theaudio. Of course, the originating Mate can inhibit transmission ofeither. The Video Ring function allows the user at the destination Mateto view the caller prior to actually receiving the call or going "OffHook". If the user wishes to confer with the originating Mate, he can doso by going to the "Off-Hook" condition. However, he can allow thesystem to stay in a continuous Ring-Back condition until the other partydisconnects.

After determining the mode by which the user at the destination Mate isnotified of an incoming call, the program flows to a decision block 1098to determine if the caller or the originating Mate has disconnected. Ifthe call has been disconnected, the program flows along the "Y" pathback to the idle state at the input of the decision block 1082 to awaitfor another incoming call. However, if the caller has not disconnected,the program flows to a decision block 1100 to determine if the Off-Hookcondition has occurred. This condition occurs when the user at thedestination Mate either lifts the handset that is connected to the SSIBor presses the "*" button on the keypad or presses the mode button. Ifthe user has not gone Off-Hook, the program flows along the "N" paththereof to a decision block 1102 to determine if the Video Ring functionhas been selected. If not, the program flows along the "N" path thereofback to the input of the decision block 1098 to determine if the calleris disconnected. If the Video Ring function has been selected, theprogram flows along the "Y" path thereof back to the input of thedecision block 1092 to determine where to display the caller.

In the Off-Hook mode, the program flows along the "Y" path from thedecision block 1100 to a function block 1104 to send a "Call Answered"message back to the Network Master. This message is generated wheneverthe destination Mate is chiming to signal an incoming call and it hasdetected the "*" keystroke, mode button pressed or sensed an Off-Hookcondition. As described above, this message is sent to the NetworkMaster and not to the originating Mate. This is facilitated by insertingthe ID "00" of the Network Master into the message as the destinationID. Once the message is sent, the program flows to a decision block 1106labeled "ACK" to indicate an "Acknowledge" decision. Each time a messageis sent from the Mate to the Network Master or a Mix in the Slave mode,an acknowledgement must be received. If the acknowledgement is notreceived, the program flows along the "N" path to a function block 1108labeled "BREAK" to indicate a termination of the call since lack of anacknowledgement indicates a problem in the network. When the properacknowledgements have been received, the program flows along the "Y"path to a function block 1110 to indicate that the call has beenreceived.

Referring further to FIG. 21, the Mate in the idle mode also looks foran Off-Hook condition. This is evidenced by the "N" path from thedecision block 1082 through decision block 1112 wherein the Mate decidesif it is Off-Hook. If not, the program flows along the "N" path back tothe input of the decision block 1082 and, if it is Off-Hook, the programflows along the "Y" path to decision block 1114. In the preferredembodiment, the only invalid digit is a "7". Any other digit is valid.However, the keys "*" and "#" are also not digits and cause the programto flow along the "N" path from the decision block 1114.

If a valid digit is selected, the program flows from the decision block1114 along the "Y" path to a function block 1116 wherein the ID of thedestination Mate is dialed. After the ID is dialed, the Mate sends a"Request to Place Call" message to the Network Master. As describedabove, the initial portion of the message contains the ID of theoriginating Mate and also the ID of the Network Master which is "00".This is followed by a specific message number which is recognized by theNetwork Master as the "Request to Place Call" message. This messagecontains the ID of the destination Mate. After sending the "Request toPlace Call" message, the program flows from the a function block 1118 toa decision block 1120 to determine if the message was acknowledged. Ifnot, the program flows to a function block 1122 to break and, ifacknowledged, the program flows along the "Y" path to a decision block1124 to determine if the system has sent back a "Response to CallRequest" message to indicate that a line is available. This message isreceived from the Network Master and indicates first if a line isavailable and, second, if the destination Mate is busy. The decisionblock 1124 determines whether the line is available. If a data link orline is not available, the program flows along the "N" path to afunction block 1126 and a "Fast Busy" signal is transmitted by the Mateto the user through the speaker or hand set. If, however, a line isavailable, the program flows along the "Y" path from the decision block1124 to a decision block 1128 to determine if the destination Mate isbusy. If the destination Mate is busy, as indicated by the NetworkMaster, the program flows along the "Y" path to a function block 1130and a "Slow Busy" signal is transmitted through the speaker or hand setto the user of the originating Mate. After either the Fast Busy or SlowBusy signal is sent, the program flows to a decision block 1132 todetermine if the user has hung up. If not, the program flows back aroundthe "N" path back to the input thereof. If the user hangs up, theprogram flows to a function block 1134 to break.

If both a line is available and the destination Mate is not busy, theprogram flows along the "N" path to a function block 1136 wherein theNetwork Master effects a connection between the originating Mate and thedestination Mate such that audio and video can be transmitted to andreceived from the destination Mate. The originating Mate connects itscamera to the video output such that video is transmitted to thedestination Mate and connects the received audio from the destinationMate to the speaker such that the Ring-Back signal can be received fromthe destination Mate. By allowing the destination Mate to generate theRing-Back, immediate aural feedback is provided as to the connection.

After connection, the program then flows to a function block 1138 toreceive the Ring Back signal, then to a function block 1140 to transmitthe video and then to a decision block 1142 to determine if the "CallAnswered" message has been received from the Network Master. The NetworkMaster sends this message after it receives the "Call Answered" messagefrom the destination Mate. Until the "Call Answered" message isreceived, the program flows around the "N" path back to the input of thefunction block 1138 to continue receiving the Ring-Back signal on theaudio path and transmitting video on the video path.

After the "Call Answered" message is received, the program flows alongthe "Y" path to the function block 1144 to indicate that the call hasbeen placed. The Mate then sends an acknowledgement back to the NetworkMaster that it has received the message and then connects the microphoneand auxiliary input to the audio SCM output. The video SCM input isconnected to the monitor and an auxiliary video is output to view theuser at the destination Mate. In addition, the camera is connected tothe viewfinder such that the user can view himself to ensure that he isin the field of view and in focus. This gives him an immediate visualfeedback of what the user at the destination Mate is viewing.

In passing messages between the Mate and the Network Master, it isnecessary to ensure that the data has arrived with no inaccuracies. Whenthe Mate transmits a message, it must receive an acknowledgement fromthe receiving device. For example, if the Mate receives a message fromthe Network Master, it has to determine if this is the first message orif it is a retry. A retry would occur if the Network Master sends aSystem message to the Mate and the Mate either does not receive themessage and acknowledge it or the Network Master does not receive theacknowledgement back from the Mate. In this instance, the Network Masterwould again send the message but would set the retry bit in the messageto indicate that this message had been previously sent. If the retry bitis not set, it recognizes this as an initial message and stores themessage and sends back an acknowledgement to the Network Master. If,however, the retry bit is set, the Mate must compare this message withthe last received message from the Network Master. If they are the same,this indicates that the acknowledged message was not received by theNetwork Master and the Mate retransmits the acknowledge message. If theNetwork Master does not receive the acknowledge message, there is afault somewhere in the return path and this is denoted as a systemfault.

When the Mate transmits a Control message, it operates similar to theNetwork Master in that it sends the first message and waits for anacknowledgement. It sets a timer to time the amount of waiting time forretransmitting the message. If no acknowledgement has been received whenthe timer times out, the retry bit is set and the message retransmitted.After three times, the Mate transmits a reset signal and returns to theidle position. If, however, the correct acknowledgement was received,the Mate then proceeds with the next step in executing the message.

From the viewpoint of the user, the Mate acts like an ordinary telephonewith the exception that video is provided. During the placing of a call,the user first goes to an Off-Hook mode which normally consists ofpressing the "*" key on the keypad of the SSIB or picking up the handsetthat is interfaced with the SSIB. The user then dials the ID of thedestination Mate, which is normally a two-digit number. For largernetworks, this number can be up to four digits. When interfacing withanother network over some form of long haul network, additional numberscan be utilized. The number of digits required for various telephonenumbers is a matter of the designers choice.

After the number has been dialed, the user will receive either a FastBusy tone, a Slow Busy tone or a Ring-Back signal. The Fast Busy toneindicates that a line is not available for interconnecting the call, aSlow Busy tone indicates that the destination Mate is busy or occupiedand a Ring-Back indicates a connection. As soon as the destination Mategoes Off-Hook, the user's video is placed on the viewfinder with thevideo from the destination Mate displayed on the monitor. Theconversation can then take place as normal.

In receiving a call, the user of the destination Mate receives a chimingsignal generated by his own Mate. If the Video Ring feature is selected,the user at the originating Mate will be displayed on the viewfinderduring the chiming. This is accomplished by having the Network Masterprovide a signal to the originating Mate to turn its camera on and thedestination Mate receiving the video and placing it on to theviewfinder. The caller then has the option to go Off-Hook or ignore thecall.

Referring further to FIG. 21, the user can select a special function bynot dialing a valid digit at the decision block 1114 such that theprogram flows to a decision block 1146 to determine if a "*" was dialed.If not, this indicates that a "#" has been dialed and the program flowsalong the "N" path thereof back to the input of the decision block 1082.If the "*" were dialed, the program flows to a function block wherein aspecial function is selected by dialing the appropriate key numbers.Special functions are special numbers that directly adjust the audio andvideo input and output of the multifunction work station. They allow theuser to connect audio and/or video to the work station and auxiliaryequipment during the idle mode, connect audio and/or video to thenetwork as well as local devices during the conference mode, directvideo switching during a conference involving three or more parties andperform diagnostic functions.

After the special function has been selected by dialing the two digitnumber in the function block 1148, the program proceeds to a functionblock 1150 to store the digits and activate the mode and then return tothe idle mode. The first digit of the special function determines thevarious input devices or the class of the functions that are selected,as outlined in Table 1.

                  TABLE 1                                                         ______________________________________                                        DIGIT        INPUT CODE                                                       ______________________________________                                        0            Blank or "turn off"                                              1            Local Processor                                                  2            Camera                                                           3            Auxiliary (either audio or video)                                4            Microphone                                                       5            Network Line                                                     6            Sound Generator                                                  7            Network Functions                                                8            Special Pre-Defined Configurations                               9            Special Diagnostics                                              ______________________________________                                    

The second digit defines the specific network or diagnostic function orthe output audio and/or video device that is utilized. For example, ifthe first digit defines an audio and/or video device or functionthereof, the second digit would define the output device as outlined inTable 2.

                  TABLE 2                                                         ______________________________________                                        DIGIT         OUTPUT DESTINATION                                              ______________________________________                                        0             Off or Default                                                  1             Monitor and Auxiliary Output                                    2             Viewfinder                                                      3             Auxiliary Audio Output                                          4             Speaker/Handset Speaker                                         5             Network line                                                    ______________________________________                                    

When selecting a special function, the user goes Off-Hook wherein a maindial tone is heard and then dials "*" and a high pitch tone is heard.The special function number is dialed and the corresponding specialfunction stored. If it is an invalid special function, a Fast Busysignal is transmitted and the user dials "#" to return to idle.

The special functions are defined as either local functions, networkfunctions or diagnostic functions. Local functions are primarilyconfined to use within the multifunction work station apart from thenetwork. These local functions can be utilized during a call or when theMate is idle. For example, if the user dials "**11" the local processorvideo is displayed on the monitor while a video tape of some similardevice connected to the auxiliary video input is watched on theviewfinder. If the user dials a "15" during a call, the local processordisplay is output on the video to the network so that the destinationMate in a two-way conference can receive this information or,alternately, all of the destination Mates in a conference. If the userdials "16", he can adjust his monitor for underscan to receive the localprocessor display that a second Mate transmits during a conference.Additonally, the user can dial "65" during a conference such thatsynthesized sounds from the local processor can be transmitted on theaudio path.

The network functions are generally utilized only during a call andbegin with the digit "7". For example, when a "71" is dialed a videoconference is locked onto a particular individual, as will be describedhereinbelow.

Diagnostic functions allow the user to check connections and install newnetworks. These are utilized for such things as switching the soundgenerator to the audio SCM line or the audio SCM line to thespeaker/handset. Additionally, any Mate can be turned into a centraltest site to service networking checks.

Mode Switch

There are basically three modes of operation for controlling thegeneration and transmission of audio and video: the idle mode, theconference mode and the holding mode. In the idle mode, the Mate canaccept an incoming call whereas a conference mode indicatescommunications on the video network. The hold mode is utilized when theuser is conceptionally in a conference mode but when no networkcommunication (i.e. SCM input/output) is enabled.

Each mode has three separate settings for the audio and videoinput/output controls. These settings are controlled by a mode switch onthe front panel of the Mate wherein sequential pressing of the modeswitch causes the settings to sequence through in a predeterminedmanner. In the preferred embodiment, there are three settings for eachof the above described modes with the mode button sequencing from thefirst setting to the second setting to the third setting and back to thefirst setting in a cyclical manner. These settings provide the user withsome additional versatility.

In the idle mode, the first setting places the output from the localprocessor onto the monitor and turns the camera off such that theviewfinder is blank. In the second setting, the camera is turned on andthe video output therefrom is displayed on both the monitor and theviewfinder. In the third setting, the user can select one of the localfunctions to define the video output of either the viewfinder or themonitor and also the audio output from the speaker. If no local functionis selected, the video from the auxiliary input is selected for displayon both the viewfinder and the monitor. By sequentially pressing themode button, three settings can be stepped through in a cyclical manner,as described above.

In the holding mode, the user's Mate is in a conference but the NetworkMaster has placed it on hold, as will be described hereinbelow. In thismode, the Mate is instructed to "loop back" its video such that theviewer is receiving his own video output. In the first setting duringthe holding mode, the local processor is displayed on the monitor andthe camera output is displayed on the viewfinder. In the second setting,the camera output is displayed on both the monitor and the viewfinder.In the third setting, either the predefined local function is selectedfor display on the monitor or the auxiliary input is displayed on themonitor with the camera output being displayed on the viewfinder. Thesethree settings, as described above, can be stepped through with the useof the mode button. During this mode, the microphone is maintained in an"on" position but no audio is being transmitted.

In the conference mode with an ongoing conference, the first settingdisplays the local processor on the monitor, displays the received videofrom the network through the SCM on the viewfinder and transmits theoutput of the camera to the SCM. In the second mode, the video from theSCM is displayed on the monitor and the camera output is displayed onthe viewfinder and transmitted to the network through the SCM. In thethird setting, either the predefined local function or the auxiliaryinput is displayed on the monitor and transmitted to the SCM and thereceived video from the SCM is displayed on the viewfinder. Themicrophone is maintained on to provide an audio data link regardless ofthe video setting.

Adding a Party-Mate

Referring now to FIG. 22, there is illustrated a flowchart for adding aparty to the conference. The flowchart begins at a START block 1152 andproceeds to a decision block 1154 to determine if a "*" has been dialed.If no "*" has been dialed, the program returns along the "N" path backto the START block 1152 to continue the conference as is. If, however, a"*" is dialed, the program flows along a "Y" path to a function block1156 to place a call in accordance with the procedure of FIG. 21 bysending a "Request to Place Call" and receiving the response to thisrequest from the Network Master. When a line is available, the NetworkMaster places all the remaining calls in "Hold". In this state, thevideo is not output from each of the Mates but, rather, is looped backsuch that the individuals in the conference are viewing their videooutput. As described above, the response from the Network Master canindicate that there is no line available. In which case, a Fast Busy isreturned or a Slow Busy signal is output if the destination Mate isbusy. During the placing of a call, the function block 1156 proceeds toa decision block 1158 to determine if an error has occurred in placingthe call, such as misdialing a number, etc. If an error is present, anerror tone is produced requiring the user to press the "#" key and theprogram then flows along the "Y" path back to the input of the decisionblock 1154. If no error is present, the procedure for placing the callcontinues on the "N" path to decision block 1160 to determine if theoriginating user has hung up. If the user has hung up, the programreturns along the "Y" path back to the input of the decision block 1154and a conference is returned to normal. If the user does not hang up,the program proceeds along the "N" path to a decision block 1162 todetermine if the call has been placed. If the call has not been placed,the program proceeds along the "N" path back to the input of thedecision block 1158 to continue the procedure for placing the call,determining if there is an error and determining if the user has hungup.

If the call has been placed, the program proceeds along the "Y" path toa function block 1164 to add the contacted party to the conferencetable. The Mate does not add the party to the conference table but,rather, the Network Master adds this party to the conference table.After the call has been placed and the party added to the conferencetable, the program proceeds to a decision block 1166 to determine if a"#" key has been depressed. If this key has not been depressed, theprogram proceeds to a function block 1168 to indicate that theoriginating Mate and the recently contacted Mate or added party remainin a two-way conference. This continues until the "#" key is depressed,at which time the program flows along the "Y" path to a function block1170 to return both parties to the conference. If the new party added tothe conference does not wish to remain in the conference, he can merelyhang up, at which time the Network Master will update the status of theconference table and remove this party from the conference table.

As will be described hereinbelow, there is a priority as to videoswitching between the Mates in the conference since only one video canbe transmitted to any one of the Mates at a given time. Each Mate isdesignated in the Network Master's tables as either a primary Mate, asecondary Mate or an M-ary Mate. The primary Mate has its videotransmitted to all of the lower order Mates in the conference with thevideo from the secondary being received by the primary. The video fromthe M-ary does not get received by any of the Mates in the conference.The rearrangement of priorities will be discussed hereinbelow withreference to the operation of the Mix and the Network Master.

Conference Lock-Mate

Referring now to FIG. 23, there is illustrated a flowchart of theprocedure for locking a single Mate as the Primary during a conference.The program is initiated at a START block 1172 and proceeds to adecision block 1174 to decide if the user has dialed "**71". If not, theprogram proceeds along the "N" path back to the input thereof until theappropriate digits have been dialed. When dialed, the program proceedsalong a "Y" path to a function block 1176 to send the Request to LockConference to the Network Master. The program then proceeds to anacknowledgement decision block 1178 to determine if the Network Masterhas received the message and, if not, the program flows along the "N"path to a function block 1180 to break back to the main program. If theacknowledgement is received from the Network Master, the program flowsalong the "Y" path to a function block 1182 wherein the Mate is in theLock Conference mode. However, the Network Master controls the operationof the network and is responsible for determining whether the requestingMate can lock a conference. This determination is made depending uponwhether another individual has already locked the conference. Forexample, if a conference is proceeding and one Mate sends a "Request toLock Conference" message to the Network Master, the Network Masterexamines a Lock Conference flag to determine if it is set or not. If itis not set, this indicates that no one has locked the conference, thusallowing the Network Master to lock the conference onto the requestingMate. However, if the flag is set, this indicates that another Mate hasalready locked the conference and this is not available. Rather, theNetwork Master merely places the requesting Mate as the primary confereeif the flag is set.

After the request has been sent and acknowledged by the Network Master,the program flows from the Lock Conference function block 1182 to adecision block 1184 to determine if the user has dialed "**70". If not,the program flows along the "N" path back to the input of the LockConference function block 1182. If the user has dialed "**70", theprogram flows along the "Y" path to a function block 1186 to send a"Request to Release Lock" message to the Network Master. The NetworkMaster receives this and sends back an acknowledgement, as indicated bya decision block 1188, and the program flows to a function block 1190 torelease lock. If the acknowledgement is not received, the program flowsalong the "N" path to a function block 1192 to break back to the mainprogram.

Voice Detect Signal-Mate

Referring now to FIG. 24, there is illustrated a flowchart illustratingthe procedure for sending the voice detect signal. The flowchart beginsat a START block 1194, then proceeds to a decision block 1196 todetermine if voice is present. If voice is not present, the programflows along the "N" path thereof to a function block 1198 to reset aninternal timer and then returns to the input of the decision block 1196to wait for voice. When the user speaks and voice is present, theprogram proceeds along the "Y" path to the input of a decision block1200 to determine if the timer is running. If the timer is not running,this indicates that the timer has not been started and the program flowsalong the "N" path to a function block 1202 to start the timer and thento a decision block 1204 to determine if the time has elapsed. If thetimer has been running at decision block 1200, the program flows alongthe "Y" path to the input of the decision block 1204.

The timer is a down counter which starts at a predetermined setting andcounts down to zero. Once the timer has counted down to zero, theprogram flows from the "Y" path to a function block 1206 to send a voicedetect signal. This voice detect signal is sent only when more than twoMates are in a conference to indicate to the Network Master that one ofthe conferees desires a higher priority. If none of the other confereesin the conference are talking such that no voice detect signal is sentfrom their Mates, the Network Master will rearrange priorities such thatthe Mate sending the voice detect signal is made the primary confereeand the prior primary conferee is made a secondary conferee with all ofthe remaining conferees being made M-ary. Until the timer has elapsed,the program will continue to flow back along the "N" path from thedecision block 1204 to the input of the Voice Present decision block1196 to ensure that voice is present for the entire duration of thetimer. If not, the timer is reset and the procedure begun again.

Data Flow-Mix

Referring now to FIG. 25, there is illustrated a schematic block diagramof the combined hardware and software for directing data flow through aMix. The schematic of FIG. 25 also illustrates the Slave and Mastertasks that operate within the Mix. The Mix is illustrated as havingeight input ports labeled SCM00 through SCM07 and a local area networkport labeled LAN. Although the SCM ports indicate the use of an SCM datalink between two units, the baseband interface 238 of FIG. 11 could beutilized in place of the SCM interconnect. In fact, all of the ports,with the exception of the LAN port, could be dedicated to basebandinterfaces rather than SCM interconnects. It is only important thataudio, video and data in the formats compatible with the Mix becompatible with the data link.

The inputs SCM00-SCM07 and LAN are input to an input interrupt I/Odriver 1220. The interrupt driver generates an interrupt whenever a byteof data is received. An interrupt software routine is operable torecognize the interrupt and then load the data into a First-In-First-Out(FIFO) data stack 1222, of which one is associated with each port. Thesoftware also contains a background subroutine, termed the "Mix Idle",which constantly checks the FIFOs 1222 and inputs it to a buffer 1224.When the buffer gets enough bytes to fill it up, the Mix Idle subroutineretrieves the data from the FIFOs 1222 and reformats them and placesthem into a linear buffer 1226, wherein one of linear buffers 1226 isassociated with each of the ports.

The linear buffers 1226 are serviced by a buffer manager routine 1228which recognizes a full buffer by an internal flag that is set by theMix Idle task. The buffer manager 1228 retrieves the information in thelinear buffer 1226 and decides where the data is to go and then routesthe data to that position. Therefore, the buffer manager 1228 is aninternal data routing software routine. The data is recognized as eitherdirected toward the Mix Slave task that is onboard the Mix, a Mastertask or for a peripheral device attached to one of its ports. Asdescribed above, each device has an ID. The Mix possesses an ID from "1"to "255", the network master or Master task possesses an ID of "00" andthe Mates possess ID's from "256" to "9999". By examining the IDs, thebuffer manager can determine where to route the data.

If the data is addressed to one of the ports, the buffer manager routesthe data to linear buffers 1232, one linear buffer provided for eachoutput port, of which there are nine.

After loading of linear buffers, they are reformatted by a buffer 1234.The buffer 1234 is similar to the buffer 1224 except that it formatsfrom the linear buffers to a First-In-First-Out format for storage inFIFOs 1236, each port having a separate FIFO 1236 associated therewith.The buffers 1224 and 1234 are essentially the Mix idle task thatrecognize when data is ready to be placed into the FIFO 1236 or takenout of the FIFOs 1222. After loading of data into the FIFOs 1236 in theappropriate formats, they are input to an output interrupt I/O driver1238. These are then connected to the ports SCM00-SCM07 and LAN outputports.

Since the data is multiplexed onto a carrier of either 70 mHz or 170mHz, the data is directly transmitted with no coupling from thereceiving device on the other end of the data link. Therefore, thisrequires some form of interrupt/buffering mechanism on the other end toreceive the data regardless of when it is transmitted. The interrupthardware and the interrupt software routines operate at a sufficientspeed and have a sufficient capacity to allow this kind of transmission.Therefore, no communication is required between two devices on eitherend of the data link regarding the time of actual transmission of dataitself since there is no chance of a "data collision" occurring on thedata link. There is always a dedicated path for data in either directionfrom one device to another.

If the buffer manager 1228 recognizes the received data as being the IDof the Mix, that is, directed toward the Mix, the data is first placedinto a buffer 1240 and then retrieved by the Mix Slave task 1230.Conversely, when the Mix Slave task generates a message for one of thedevices on its ports, it first places the data into an outgoing buffer1242 and then this data is input to the buffer manager 1228. The buffermanager 1228 then recognizes where this data is to be routed, whichaction is performed.

In addition to the output ports for the SCM's and the various peripheraldevices attached thereto and the data link to the LAN, a Mix Master task1244 is also resident on board the Mix. Data received by the buffermanager 1228 and directed toward the Mix Master task 1244 is first inputto a buffer 1246 and then retrieved therefrom by Mix Master task 1244.Conversely, data being output by the Mix Master task 1244 is first inputto a buffer 1248 and then input to the buffer manager 1228. Datatransfer from the buffer manager 1228 to the Mix Master task 1244utilizes a memory to memory transfer.

In operation, the Mix Master task 1244 is essentially a virtual tenthport for the Mix. The buffer manager 1228 maintains a table regardingthe status of all of the ports attached thereto. Therefore, it knows theID of all Mates or Mixes attached to the SCM ports and it also knows towhich port the Network Master is connected. As described above, there isalways a direct data link between any Mix and the Network Master.

If the Network Master is on the LAN port, a message with an ID of "00"is always directed to that port. However, if the on board Mix Mastertask 1244 is activated and is operating as a Network Master, thisinformation is contained in the internal tables of the buffer manager1228 and all messages received for the Network Master by the buffermanager 1228 are routed through the virtual tenth port to the internalMix Master task 1244. Of course, if the Mix Master task 1244 isinactive, the Network Master occupies one of the other nine ports. Thisis also true with information received from the Mix Master task 1244,since the buffer manager 1228 must first receive the data and thendecide where the data is to be routed. It can either be routed to theMix Slave task 1230 or to one of the other peripheral devices on theports. However, there is one slight variation to messages received fromthe Mix Master task that are directed towards Mates on another Mix thatis connected to one of the SCM ports. Once the buffer manager 1228receives the message from the Mix Master task 1244 when acting asNetwork Master, it scans its memory and determines that the destinationMate is not directly attached to any of its ports. When this situationoccurs, a data field is provided in the message header which containsinformation regarding which port the buffer manager 1228 is to transmitthe data. This data field is referred to as the Task ID. This portion ofthe header is only utilized by the resident Mix Master task 1244 whentransferring data through the associated buffer manager 1228. Once thebuffer manager 1228 recognizes that the destination Mate is not directlyattached to any of its ports, it then examines the information containedin the task ID to determine which port the data is to be transmitted.

The software of the Mix operates under the control of a Task SchedulingProgram. This utilizes the occurrence of events to increment betweenvarious tasks or programs in the Mix instead of a time slice. Therefore,each program is completely executed before the next task occurs.Essentially, the task scheduler calls the subroutine for the Mix Idletask then calls the Mix Slave task and then calls the Mix Master task.Of course, if the Mix Master task is not activated, this is immediatelybypassed. The task scheduler has an internal timing routine thatprevents a given task from "locking" such that it can return to the taskscheduling program and then reset everything.

Referring now to FIG. 26, there is illustrated a diagrammatic view ofthe video conferencing network which more clearly illustrates the dataflow therethrough. Like numerals refer to like parts in the variousfigures. The video conferencing network which is comprised in the firstlevel of a single Mix 66 and in higher levels with multiple Mixes, isbasically comprised of an audio/video plane for carrying the audio/videoinformation and comprised of a switch 1201 and also a data plane whichis comprised of a data manager 1203. The audio/video and datainformation are all transmitted between the Mate 10 and the networkthrough the SCM cable 16. At the Mate 10, the interface to the SCM cable16 is the SCM 51 whereas the network or the Mix is interfaced with theSCM cable 16 through the SCM 68. At the SCM 68, audio and videoinformation is passed to the audio/video switch 1201 in the audio/videoplane through an audio/video path 1205. In a similar manner, the data istransferred to the data manager 1203 through data paths 1207. The datamanager 1203 therefore interfaces with the Mates through its data portson lines 1207. This data is then transformed into a format compatiblewith the transmission medium which, in the preferred embodiment, is theSCM cable and frequency division multiplexing.

The data manager 1203 has a memory 1209 interfaced therewith and alsohas a slave device 1211 interfaced with one port thereof and a NetworkMaster (NM) 1213 interfaced with another port thereon. The slave 1211 isoperable to control the audio/video switch 1201 to configure theinterconnections therein. The Network Master 1213 has a system memory1215 associated therewith. The data manager 1203 is operable to receivedata on any of the ports from either the lines 1207 interfaced withMates 10 or from the port to which the Network Master 1213 is connectedor the port to which the slave 1211 is connected. The data manager 1203does not process the data but merely routes the data to the appropriateposition after recognition thereof. For example, if a Mate 10 desires toconverse with the Network Master 1213, it transmits a message to thedata manager encoded with the Network Master's ID. The data managerreceives the message and examines the memory 1209 to determine whichport the Network Master is on. The data is then routed to this portwhich is the port that Network Master 1213 resides on. In a similarmanner, the Network Master 1213 can transmit a message to the datamanager 1203 indicating that it wishes to communicate with one of theMates 10. This is also encoded in the message. The data manager 1203then examines the memory 1209 to determine on which of its network portsthe Mate 10 is connected and routes the data thereto.

The slave 1211 connected to the data manager 1203 is operable, as willbe described hereinbelow, to update the memory 1209 by determining thestatus of all of the items on the network ports. Slave 1211 "polls" thevarious ports by sending messages to the devices attached to theseports. For example, if the slave 1211 wishes to converse with a port onwhich a Mate 10 resides, it sends a message to the data manager 1203with information regarding the location of this port. The data manager1203 then routes this message to the port and it is routed to the Mate10 attached to that port. The Mate 10 then responds with a message tothe data manager 1203 that is directed toward the slave 1211. The datamanager then routes this message to the slave 1211.

The Network Master 1213 can also address the slave 1211 to determine theswitching configuration of the audio/video switch 1201. This is astandardized message which is directed only to the slave 1211.Therefore, the slave 1211 not only polls the various ports to update thememory 1209 but also reconfigures the switch 1201.

The video conferencing network is made up of the data plane and theaudio/video plane with the data plane providing network control and theaudio/video plane providing an audio/video path for videocommunications. In addition to accommodating video communications, thenetwork of the present invention is also operable to exist in parallelwith an existing system or network. As described above, the localprocessors 42 are interconnected with the SCM's 51 through the line 47.This allows baseband information to co-exist with video conferencinginformation on the SCM cable 16. However, at the SCM 68, the basebandinformation is separated and transmitted on the line 77 to a Local AreaNetwork 1217. This Local Area Network 1217 is operable to transferbaseband information between the local processors 42 and otherperipheral Local Area Network devices that may be attached to the LocalArea Network 1217. The baseband data transfer between devices on theLocal Area Network 1217 is, as described above, separate and distinctfrom any data or audio and video transmitted on the video conferencingnetwork. By allowing a separate and distinct Local Area Network 1217 orany other type of network to share the SCM cables 16 with the videoconferencing network, it is only necessary to provide one cable from acentral location to a remote location to provide two networkingfunctions. For example, in some office facilities, an existing localarea network is utilized for carrying on the basic functions in theoffice environment. By utilizing the configuration of the presentinvention, it is possible to install a video conferencing networkwithout requiring installation of new communication links between acentral area and the remote points.

Referring now to FIG. 27, there is illustrated a side view of theNetwork of FIG. 26 illustrating only two Mates and also the use of theLocal Area Network 1217 as the control medium for the Network Master.Like numerals refer to like parts in the two Figures. In the system ofFIG. 27, the Network Master port is interfaced to the LAN 1217 through aLAN interface 1219 and the LAN 1217 is interfaced with the NetworkMaster 1213 through a LAN interface 1221. The LAN interfaces 1219 and1221 allow the Network Master 1213 to operate in an identical manner tothat of the system of FIG. 26. For example, if the Network Master 1213desires to send a message to one of the ports on the data manager 1203,it places baseband data onto the LAN 1217 and this data is processedlike any baseband data on a network. The video conferencing network isessentially a peripheral device that is attached to the LAN 1217 and itreceives data in the same manner that the local processors 42 receivedata. This data is recognized by the LAN interface 1219 as beingdirected toward a video conferencing network and the data is then inputto the data manager 1203 for routing. A message from the data manager1203 to the Network Master 1213 is similarly converted to baseband andthen back to a format compatible with the Network Master 1213.

Referring now to FIG. 28, there is illustrated a diagrammatic view ofthe data manager 1203 which is divided up into three subdata managers1223, 1225 and 1227. Each of the subdata managers 1223-1227 has a datalink provided therebetween. This configuration constitutes the dataplane for the three mix configuration of FIG. 7. Each of the subdatamanagers 1223-1227 has two Mates 10 attached thereto through SCM cable16. The SCM's 51 and 68 are not shown for simplicity purposes.

Subdata manager 1223 has associated therewith a Network Master 1229 anda slave 1231. The subdata manager 1227 has a Network Master 1233 and aslave 1235 associated therewith. The subdata manager 1225 has a slave1237 and a Network Master 1239 associated therewith. The slaves 1231,1235 and 1237 operate similar to the slave 1211 except that theydistribute their functions. However, the Network Masters 1229, 1233 and1239 do not act in a distributed manner. Rather, as described above,only one of the Network Masters is active at a given time. The activeNetwork Master is illustrated as the Network Master 1229 with theremaining Network Masters 1233 and 1239 shown with dotted lines toindicate inactive status.

When a message is sent from one of the Mates 10 on the subdata manager1227, the subdata manager 1227 looks into its internal memory todetermine where the message should be sent. It recognizes that itsNetwork Master 1233 is not active and that it must route the message tothe subdata manager 1223. When the subdata manager 1223 receives thismessage, it recognizes that it is to be routed to the Network Master1229 on the associated port. In routing data from a peripheral device tothe Network Master, it is only necessary to designate one of its portsas the Network Master port. Since the ID of "00" indicates the NetworkMaster, the subdata managers 1227 only need to transmit it to the portdesignated as the Network Master. Since the message will always berecognized by any of the subdata managers, it can always be routed tothe correct port which is designated as the Network Master port.

In transmitting a message from the Network Master to a designatedNetwork port, it is necessary to determine the data path for thistransmission. For example, if the Network Master 1229 which is activedesires to transmit a message to one of the Mates 10 on the subdatamanager 1227, it is first necessary to transmit the message to subdatamanager 1223 and encode in that message the port to which the data is tobe routed. The shortest path would be the direct path between thesubdata manager 1223 and the subdata manager 1227. However, the messagecould go through the subdata manager 1225. It is only necessary thatwhen a subdata manager receives a message that is to be directed toanother subdata manager, the message be encoded with the informationregarding the port to which it is to be sent. Depending upon the size ofthe network and the number of subdata managers, the amount of memory andprocessing capability of the Network manager can become unwieldy.

Servicing Call Request-Network Master

Referring now to FIG. 29, there is illustrated a flowchart for servicingof a call from the Mate by the Network Master. The Network Master caneither be resident on the associated Mix, a separate central processoron a LAN or the Master task running in a separate Mix. The program isinitiated at a START block 1256 and proceeds to a decision block 1258 todetermine if a "Request to Call" message has been received from a Mate.Until this message is received, the program flows back along the "N"path to the input thereof. When the message has been received, theprogram flows along the "Y" path to a function block 1260 to save the IDof the Mate. As described above, the Mate ID is the originating ID withthe ID of the Network Master "00" being the destination ID coded intothe Mates message.

Once the ID of the requesting Mate has been stored, an acknowledgementis sent back, as indicated by a function block 1262. The program thenflows to a function block 1264 to check destination availability. Thisdestination availability is stored in internal tables in the NetworkMaster. Therefore, the Network Master does not have to go out and pollthe various Mates and associated Mixes to determine if either the Mateis already associated with a conference or if there is a path availableto the Mate. The program then flows to a decision block 1265 todetermine if the Mate is available for a conference. If not, thisindicates that the Mate is already involved in a conference and theprogram flows along the "N" path thereof to a function block 1266 tosend a "Slow Busy" message back to the requesting Mate. The program thenflows to a decision block 1268 to determine if a "Request to Terminate"message has been received from the requesting Mate. When this message isreceived, a response is sent back to the requesting Mate and the programflows along the "Y" path to a function block 1270 labeled "Quit" toreturn to the main program. However, until the Request to Terminatemessage is received, the program flows back to the input of the decisionblock 1268 to await receipt thereof.

If a destination Mate is available, the program flows along the "Y" pathfrom the decision block 1265 to a decision block 1272 to determine ifthere is an available data path between the originating Mate and thedestination Mate. If not, the program flows along the "N" path to theinput of a function block 1274 to send a "Fast Busy" message back to theoriginating Mate and then the program flows to a decision block 1276 todetermine if a "Request to Terminate" message has been sent by theoriginating Mate. Until this message is received, the program flows backalong the "N" path to the input thereof. When the message has beenreceived, the program flows along the "Y" path to a Quit function block1278.

If a path is available, the program flows along the "Y" path fromdecision block 1272 to a function block 1280 and a connect command issent from the Network Master to the Mix Slave with the informationregarding the switching interconnection for each Mix in the networkinvolved in the data link. Depending upon the complexity of the datalink, a number of Mixes may be involved since a direct path may not beavailable. The program then flows to a decision block 1282 to await anacknowledgement from the Mix Slave. The program loops around along the"N" path back to the input thereof until this acknowledgement has beenreceived. When the acknowledgement has been received, the tables in theNetwork Master are updated and then the program flows along the "Y" pathto a function block 1284 to send an "Incoming Call Request" message tothe destination Mate. As described above, the audio and video paths areestablished prior to sending an "Incoming Call Request" message to thedestination Mate. Therefore, when the "Incoming Call Request" message issent to the destination Mate, an audio and video path is alreadyestablished, thus providing the destination Mate with the ability toselectively receive any audio and data information transmittedtherealong. However, the originating Mate may or may not be transmittingaudio and video.

After sending the "Incoming Call Request" message, the program flows toa decision block 1286 to determine if a "Terminate" message has beenreceived from the originating Mate indicating that he has hung up. Ifso, the program flows along the "Y" path to a quit block 1288 and, ifnot, the program flows to the input of a decision block 1290. Thedecision block 1290 determines whether a response from the destinationMate has been received. If not, the program flows back to the input ofthe decision block 1286 along the "N" path thereof to await for the"Terminate" message and if a response has been received, it flows alongthe "Y" path to the input of a decision block 1292.

The decision block 1292 determines whether the destination mate hasaccepted the call by sending a "Call Answered" message. If the call isnot accepted, the program flows along the "N" path to a function block1294 and a "Slow Busy" message is sent back to the originating Mate andthen the program flows to a flip-flop 1296. This path indicates that forsome reason there was no acknowledgement or response received from theMate although the tables of the Network Master indicated that it shouldbe free.

If a call is accepted, the program flows along the "Y" path from thedecision block 1292 to a function block 1298 and a "Path Available"message is sent to the originating Mate. The program then flows to anacknowledgement decision block 1300 and an associated quit block 1302and then to a decision block 1304 to determine if the call is answered.As soon as a response is received from the destination Mate that it hasreceived the "Incoming Call" message, it begins the Ring-Back to theoriginating Mate along the audio path and also begins chiming at thedestination Mate itself. This will continue until either the originatingMate sends a "Termination Request" message, as indicated by the decisionblock 1306 and a quit block 1308.

When the destination Mate has answered the call, the program flows alongthe "Y" path from the decision block 1304 to a function block 1310 tosend the "Call Answered" message back to the originating Mate. Theprogram then flows through an acknowledgement decision block 1312 andassociated quit block 1314 to a return block 1316.

During communication between a Mate and the Network Master, the Mate'smessages are first sent to the buffer manager on the associated Mix andthen routed to the port on which the Network Master resides or to theinternal Mix Master task which constitutes the Network Master.Therefore, with the use of the buffers between all of the ports, thebuffer manager can control the flow of information between the variousports such as from the Network Master to the Mate or from the Mix Slaveto the Mates or even from the Mix Slave to the Network Master. It isnecessary to have this type of control to prevent any attempt totransmit two messages on the same port. For example, the buffer managerwould prevent the Network Master from sending a message directly to adesired one of the Mates at the same time that the Mix Slave wishes tocommunicate with the same Mate.

Add Party to Conference-Network Master

Referring now to FIG. 30, there is illustrated a flowchart for theprocedure of adding a party to a conference by the Network Master. Theprogram is initiated at a START block 1320 and proceeds to a decisionblock 1322 to determine if a "Request to Call" message has been receivedfrom one of the Mates in the conference. The Network Master maintains atable of all ongoing conferences and conferees associated therewith. Thedecision block 1322 represents only the procedure for servicing theMates within the conference. Therefore, if a "Request to Call" messagehas not been received, the program flows along the "N" path to adecision block 1324 to determine if a "Request to Terminate" message hasbeen received from any of the Mates in the conference. If not, theprogram proceeds along a "N" path back to the input of the decisionblock 1322. However, if a "Request to Terminate" has been received byone of the Mates in the conference, the program flows along the "Y" pathto a function block 1326 to update the internal tables of the NetworkMaster to the current status of the conference and then returns to theinput of the decision block 1322.

When a "Request to Call" message is received from one of the Mates inthe conference, the program proceeds along the "Y" path from thedecision block 1322 to a function block 1328. In the function block1328, the Network Master determines if a path and the destination Mateare available, as described above with reference to blocks 1264-1278 ofFIG. 29. If a path is available, the program flows to a function block1330 to send a switch command to the Mix Slave to set up a communicationpath. The program then flows to a function block 1332 and a "HoldMessage" is sent to all of the Mates in the conference to place them onhold. As described above, each of the Mates in the conference replieswith a response and then loops back its video from the camera to themonitor and the viewfinder. This video is not transmitted through theSCM but, rather, through the video multiplexers in the Mate.

After all of the other Mates in the conference have been placed on hold,the call is placed in accordance with the procedure of FIG. 29, asindicated by a function block 1334. The program then proceeds to adecision block 1336 to determine if the call has been placed. If thecall has been placed, the program proceeds along the "Y" path to afunction block 1338 to update the conference table and add the partythereto. If the call is not placed due to a busy signal or anunavailable line, the program proceeds along the "N" path to bypass thefunction block 1338.

After the call has either failed to place or the party has been added tothe conference, the program proceeds to a decision block 1340 to waitfor a "Request to Unhold the Conference" message from the originatingMate. Until this request has been placed, the originating Mate and thenew party are maintained in a two-way conference, as indicated by a loopback along the "N" path thereof. When the originating Mate sends the"Request to Unhold" message, the program then flows along the "Y" pathto a function block 1342 to redefine the priority of conferees in theconference. As described above, there is a primary conferee and asecondary conferee with the remaining conferees being designated asM-ary. The primary conferee has his video output connected to theremaining conferees with the video of a secondary connected to hismonitor. All M-arys receive the video from the primary.

In setting the priority, the prior primary is reduced to a secondary, asindicated by the function block 1342 and the prior secondary and all theremaining M-arys to M-arys, as indicated by function block 1344. Theoriginator is then set to a primary, as indicated by a function block1346. After the priority of the conferee is determined, the programflows to a function block 1348 to send a "Request to Unhold" message tothe conferees and then the program flows to the return block 1350.

Conference Priority--Network Master

Referring now to FIG. 31, there is illustrated a flowchart for alteringthe priorities of conferees during a conference. This is essentiallyaccomplished with the VOX detect circuit which provides a signalindicating that one user is talking for a duration of time that exceedsapproximately 750 milliseconds. If no other individual is talking forthis extent of time, the Mate from which the Voice Detect signal isoriginated will become the primary in the conference.

The program is initiated at a START block 1352 and then proceeds to adecision block 1354 to determine if a VOX Detect signal has beenreceived. If not, the program flow loops back around to the input alongan "N" path. When the VOX Detect signal is received, the program flowsalong the "Y" path to a decision block 1356 to determine if there aremore than two Mates in the conference. Although not shown, the programrequires that more than one half second elapse between the last receivedVOX before flowing to block 1356. If not, there is no need to determinepriority and the program flows back around along the "N" path. However,when more than two Mates are in the conference, the program flows alongthe "Y" path to a decision block 1358 to determine if the VOX Detectsignal is from the primary. If so, the program flows along the "Y" pathand loops back around to the beginning of the program. However, if theVOX detect signal is received from other than the primary, the programflows along the "N" path to a function block 1360 to determine if thesignal is from the secondary. If the signal is from the secondary, theprogram flows along the "Y" path to a decision block 1361 to determineif the conference is locked. If so, the program flows to a return block1363 and, if not, the program flows to a function block 1362 to changethe secondary to the primary and then to a function block 1364 to changethe primary to secondary. However, if the Voice Detect signal is from anM-ary, the originating M-ary conferee is changed to the secondary, asindicated by a function block 1366 and then the secondary is changed toM-ary, as indicated by a function block 1368. The primary remains in theprimary position. After rearranging the tables of the network master,the program proceeds to a Return block 1370.

Although not described with reference to FIG. 31, anyone can lock theconference such that all of the remaining conferees receive video fromhim. This only requires the input of one of the special function codesdescribed above.

Status Check-Mix

Referring now to FIG. 32, there is illustrated a flowchart for thecommunication between the Mix Slave and its ports which allows the MixSlave to maintain an updated status. The program is initiated at a STARTblock 1372 and proceeds to a function block 1374 wherein "Keep Alive"signals are sent to selected ports. These are "Request For Status"messages from the Mix Slave to the Mate, to another Mix or to theNetwork Master. This is followed by the Acknowledgement signals from theMate back to the Mix Slave and then followed by a message as to the Matestatus. The program then flows to a decision block 1376 to determine ifthere has been a status change. Since the Mix Slave maintains a table asto the status of all devices attached to its ports, this new data isalways compared with the old data. If there has not been a statuschange, the program proceeds along the "N" path to a function block 1378to increment the selected port and check the status of another port andthen returns to the input of the function block 1374 to send "KeepAlive" signals to that port. However, if the status has changed, theprogram proceeds from the decision block 1376 along the "Y" path to afunction block 1380 wherein a "Reconfiguration Request" message is sentto the Network Master. This "Reconfiguration Request" message isutilized by the Mix Slave to apprise the Network Master of currentstatus on the ports. This is placed into the tables of the NetworkMaster to maintain a centralized information bank as to network status.The program then flows to the function block 1378 to increment the portand determine the status of the next sequential port.

Mix Slave-Received Messages

Referring now to FIG. 33, there is illustrated a flowchart for theoperation of the Mix Slave and the various messages or requests itreceives. The program is initiated at a START block 1382 and thenproceeds to a decision block 1384 to determine if it has received a"Request Configuration" message from the Network Master. If so, theprogram flows around the "Y" path to a function block 1386 to send theconfiguration data to the Network Master and then returns. If not, theprogram flows along the "N" path to determine if it is to reconfigureits audio and data paths. This is determined by a request from theNetwork Master to add or delete parties from a conference or toterminate a conference. If this command has been received from theNetwork Master, the program flows from the decision block 1388 to afunction block 1390 along the "Y" path to receive the message from theNetwork Master and effect the connection in accordance therewith. Theprogram then returns back to the main flow. If the message is not toreconfigure the switches, the program flows along the "N" path to adecision block 1392 to determine if a "Request for Reset" is required.This message is sent when the Network Master has encountered a problemin synchronizing with the slave. If the reset message is received, theprogram then flows to a function block 1394 to resynchronize with theNetwork Master. Resynchronization with the Network Master requiresclearing of all buffers between the Mix Slave and the Network Master andinitiating the transmission again. However, none of the status tables ornetwork information is altered since only the buffers related with thecommunication between the Network Master and that particular Mix Slaveis involved. In a similar manner, a "Reset Request" can occur between aNetwork Master and a Mate indicating some communication problemtherebetween. The buffers concerned with the communication between thesetwo devices on that particular port are then cleared and transmissionbegun again.

After it is determined if there is a "Reset" message received, theprogram then flows to a decision block 1396 to determine if there is a"Request Error Status" message received, in which case the "ErrorStatus" is sent, as indicated by a function block 1398. If no "ErrorStatus" is requested, the program flows to a decision block 1400 todetermine if any "Keep Alive" messages have been received. These aremessages from other devices such as Mixes and the Network Master todetermine the status of its ports. If no "Keep Alive" messages have beenreceived, the program flows to a Return block 1402. However, if a "KeepAlive" message has been received, the Mix Slave must determine where the"Keep Alive" message is from. In a decision block 1404, it is determinedwhether it is received from a Mix. However, if the "Keep Alive" messageis from a Mix, the program flows along the "Y" path to a decision block1406 to determine if the ID of the Mix is lower than the ID of the MixSlave. If the ID of the Mix sending the "Keep Alive" signal is lowerthan the present Mix Slave ID, the program flows to the return block1402. However, if the "Keep Alive" signal is from a Mix that has an IDthat is higher than itself, it proceeds to a function block 1408 toupdate the Network Master port. In updating the Network Master port, theMix Slave must first determine if another port already has a NetworkMaster thereon. If so, the program then flows to the Return block 1402.If there is no Network Master on any of the ports, the Mix Slave looksto see if its internal Mix Master task is activated. If so, itrecognizes itself as the Network Master. However, if its internal MixMaster task is not activated and its ID is lower than that of the Mixfrom which the "Keep Alive" signal is received, it proceeds to activateits internal Mix Master task, thus becoming the Network Master.

For example, if there are three Mixes in a network with the ID's "01","02" and "03", there will be three Mix Master tasks available forcontrolling the system. However, only one Mix Master task can operate asNetwork Master, with the other two being deactivated. To determinepriority, the system of the present invention predefines the NetworkMaster as being the Mix with the lowest ID. Upon powering up of thesystem, each of the Mix Slaves activates its internal Mix Master task.The Mix Slaves then send "Keep Alive" signals to each of the other Mixesin the system and receive responses therefrom. In this manner, they candetermine first that a Mix is attached to a given port and second the IDof that Mix. If the ID is lower than its ID, it deactivates its internalMix Master task. However, if the ID is higher, it allows the Mix Mastertask to remain activated and becomes the Network Master. In addition, italso receives "Keep Alive" signals from the other Mixes and respondsthereto such that they can also determine whether to inactivate theirinternal Mix Master tasks.

In a system where the Network Master is disconnected from the system,each of the Mixes determines this event through the use of "Keep Alive"signals to its various ports. Regardless of the device that is removedfrom this port, the Mix Slave is aware of this event. When it determinesthat both the device has been removed from its port for reasons such aspower down, etc. and that it was a Network Master, the Mix Slaveautomatically powers up its internal Mix Master task. The remainingMixes in the system do the same and then they proceed to determine whichMix Master task takes over the system as Network Master. However, when aNetwork Master is removed from this system, all of the switches, buffersand memory are cleared to an initial power up state such that allinformation is lost. Therefore, any communications that are ongoingduring the loss of a Network Master are removed and all thecommunication paths must be reaccessed.

In summary, there has been provided a video conferencing system whichprovides a plurality of remote video conferencing terminals which areconnected to a switching network for interconnecting both audio andvideo paths between two or more terminals for either a two-wayconference or a multiway conference. Each of the remote terminals is incommunication with a network controller which determines all data flowand network priority with respect to placing of calls, etc.

The remote terminals are arranged in small groups, each connecteddirectly to a switching network which is controlled by a Slavecontroller. The Slave controller controls the associated switch andmaintains an updated status of all of the remote terminals associatedtherewith. Each of the Slave controllers is directly connected to thenetwork controller through a dedicated data path to receive data fromthe network controller and route it to the appropriate terminal.Alternately, the Slave controller can receive the instruction itself forexecution such as updating the status of its ports or routing datathrough its switches. Each of the switching networks associated with theSlave controllers possesses a direct data path to each of the otherswitching networks in the overall Network.

The network controller is a subroutine task that is provided for in eachof the switching networks associated with the groups of remoteterminals. However, only one of the internal network controllerssubroutines can operate and this is determined by a predefined priority.Therefore, if one of the switching devices is deactivated, a networkcontroller can be established with the remaining units in a Master/Slaverelationship.

Although the preferred embodiment has been described in detail, itshould be understood that various changes, substitutions and alterationscan be made therein without departing from the spirit and scope of theinvention as defined the appended claims.

What is claimed is:
 1. A conferencing system for communicating betweenremote stations, comprising:a transmission medium associated with eachof the remote stations for carrying audio signals; microphone meansdisposed at each of the remote stations for receiving audio informationfrom said associated transmission medium; speaker means at each of theremote stations for outputting audio signals to said associatedtransmission medium; said associated transmission providing a feedbackpath between said microphone and speaker means; communication link meansfor providing a first audio path between said microphone means at one ofthe remote stations and said speaker means at selected ones of theremaining remote stations; said communication link means providing asecond audio path between said microphone means at each of the selectedremaining remote stations and said speaker means at the one remotestation, said first and second audio paths being isolated; and loop gainmeans for inserting loss in the audio path at each of the remote pointssuch that the audio level of received audio from said communication linkmeans that is transmitted from said speaker means to the associated onof said microphone means and transmitted to said communication linkmeans is decreased to prevent oscillation, said inserting means adjustedas a function of the input signal level to said microphone means, saidinserting means comprising:level control means for selecting a desiredlevel for output from said speaker means; microphone loss means disposedbetween said microphone means and the audio output of the associatedremote station for selectively inserting a first predetermined amount ofloss into the audio loop; speaker loss means disposed between saidspeaker means and the input of the associated remote station forselectively inserting a second predetermined amount of loss into theaudio loop; voice detect means for generating a voice detect signal whenthe input level to said microphone means exceeds a predeterminedthreshold; and loop control means for activating said microphone lossmeans in a first mode and activating said speaker loss means in a secondmode, said loop control means operating in said first mode in theabsence of said detect signal and in said second mode when said voicedetect signal is present, said control means inhibited from operating ineither said first or second mode when the desired output level of saidlevel control means is less than a predetermined level threshold; saidfirst predetermined amount of loss in said microphone loss means being afunction of the setting of said level control means such that the lossinserted increases as the desired level set by said level control meansincreases above said predetermined level threshold.
 2. The conferencingsystem of claim 1 wherein said transmission medium is air.
 3. Theconferencing system of claim 1 wherein said transmission medium has aninherent loss associated therewith.
 4. The conferencing system of claim1 wherein said microphone means is operable to receive audio informationfrom an external source transmitting through said associatedtransmission medium, said microphone means summing the audio informationfrom said external source and the output of said speaker means.
 5. Theconferencing system of claim 4 wherein said external source is thespoken voice of an individual.
 6. The conferencing system of claim 1wherein said communication link comprises:centralized means forreceiving the audio information from each of the remote stations in theconference and transmitting audio to each of the remote stations in theconference; and summing means associated with each of the remotestations for summing the audio from the remaining ones of the remotestations in the conference for transmission to the associated one of theremote stations.
 7. The conferencing system of claim 1 wherein theincreased signal level to said microphone means that activates saidvoice detect means is derived from an external voice signal summed withthe output of said speaker means at said microphone means.
 8. Theconferencing system of claim 1 and further comprising:handset means forincreasing the audio path loss through said transmission medium betweensaid microphone means and said speaker means; and means for inhibitingsaid loop control means from activating said microphone loss means orsaid speaker loss means when the audio path loss between said microphonemeans and said speaker means is increased.
 9. The conferencing system ofclaim 1 and further comprising means for varying said first and secondpredetermined amount of loss in said microphone loss means and saidspeaker loss means as a function of the number of remote stations in theconference such that the first and second predetermined losses areincreased for each added remote station.
 10. A conferencing system forcommunicating between remote stations, comprising:a transmission mediumassociated with each of the remote stations for carrying audio signals;microphone means disposed at each of the remote stations for receivingaudio information from said associated transmission medium; speakermeans at each of the remote stations for outputting audio signals tosaid associated transmission medium; said associated transmission mediumproviding a feedback path between said microphone and speaker means;communication link means for providing a first audio path between saidmicrophone means at one of the remote stations and said speaker means atselected ones of the remaining remote stations; said communication linkmeans providing a second audio path between said microphone means ateach of the selected remaining remote stations and said speaker means atthe one remote station, said first and second audio paths beingisolated; and loop gain means for inserting loss in the audio path ateach of the remote points such that the audio level of received audiofrom said communication link means that is transmitted from said speakermeans to the associated one of said microphone means and transmitted tosaid communication link means is decreased to prevent oscillation, saidinserting means adjusted as a function of the input signal level to saidmicrophone means; said loop gain means comprising: means for sensing theoutput of said microphone means and generating a detect signal when theinput level to said microphone means exceeds a predetermined audiothreshold; first loss means for decreasing the output level of saidmicrophone means; second loss means for decreasing the output level ofsaid speaker means; and control means for activating only said secondloss means in response to generation of said detect signal to reduce theoutput level of said speaker means to introduce loss into the audioloop, said control means activating only said first loss means in theabsence of said detect signal to reduce the output signal of saidmicrophone means to introduce loss into the audio loop such that apredetermined amount of loss is maintained between the received audiosignal level and the transmitted audio signal level in the associatedone of the remote stations; said control means further comprising meansfor varying the predetermined decrease in said first and second lossmeans as a function of the number of remote stations in the conferencesuch that additional loss is inserted as the number of remote stationsis increased; said first and second loss means decreasing the outputlevel of said microphone and speaker means, respectively, by apredetermined amount.
 11. The conferencing system of claim 10 andfurther comprising:means for selecting a desired output level from saidspeaker means; and means for deactivating said first and second lossmeans below a predetermined desired output level such that no loss isintroduced into the audio loop at the associated one of the remotestations when said selecting means is set below said predetermineddesired output level.
 12. The conferencing system of claim 10 whereinsaid detect signal is generated when the output of said microphone meansexceeds said predetermined threshold after a predetermined period oftime.
 13. An audio conferencing network, comprising:a plurality ofremote stations; central switching means for interconnecting select onesof said remote stations in a conference; communication link meansdisposed between each of said remote stations and said switching meansfor providing a full duplex audio path for receiving audio from theassociated one of said remote stations on a first path and transmittingaudio thereto on a second path, said first and second paths isolatedfrom each other; summing means associated with said switching means andeach of said remote stations for receiving and summing audio from theremaining ones of said remote stations in the conference fortransmission to the associated one of said remote stations; microphonemeans associated with each of said remote stations for receiving audiosignals from an associated transmission medium and outputting thereceived audio to said first path of the associated one of saidcommunication link means; speaker means associated with each of saidremote stations for receiving audio from said second path of theassociated one of said communication link means and outputting thereceived audio to said associated transmission medium; said first andsecond paths in said communication link means between said remotestations in the conference and the audio path through said associatedtransmission medium forming an audio loop; and means associated witheach of said remote stations for selectively increasing the loss in saidaudio loop at the associated one of said remote stations to prevent saidaudio loop from oscillating due to variations in the loss in theassociated one of said transmission mediums and the level of audioreceived from external sources in said associated transmission medium,said means for increasing the loss of said audio loop has the lossincrease determined by the number of said remote stations in theconference.
 14. The conferencing network of claim 13 wherein saidcentral switching means comprises a plurality of audio multiplexers,each associated with one of said remote stations and operable to receiveaudio from said first path in each of said communication link means andselectively output the ones of said first paths associated with the saidremote stations in the conference to said summing means.
 15. An audioconferencing network, comprising:a plurality of remote stations; centralswitching means for interconnecting select ones of said remote stationsin a conference; communication link means disposed between each of saidremote stations and said switching means for providing a full duplexaudio path for receiving audio from the associated one of said remotestations on a first path and transmitting audio thereto on a secondpath, said first and second paths isolated from each other; summingmeans associated with said switching means and each of said remotestations for receiving and summing audio from the remaining ones of saidremote stations in the conference for transmission to the associated oneof said remote stations; microphone means associated with each of saidremote stations for receiving audio signals from an associatedtransmission medium and outputting the received audio to said first pathof the associated one of said communication link means; speaker meansassociated with each of said remote stations for receiving audio fromsaid second path of the associated one of said communication link meansand outputting the received audio to said associated transmissionmedium; said first and second paths in said communication link meansbetween said remote stations in the conference and the audio paththrough said associated transmission medium forming an audio loop; andmeans associated with each of said remote stations for selectivelyincreasing the loss in said audio loop at the associated one of saidremote stations to prevent said audio loop from oscillating due tovariations in the loss in the associated one of said transmissionmediums and the level of audio received from external sources in saidassociated transmission medium, said means for selectively increasingthe loss in said audio loop comprising:level control means for selectinga desired level for output from said speaker means; microphone lossmeans disposed between said microphone means and the audio output of theassociated one of said remote stations for selectively inserting a firstpredetermined amount of loss into said audio loop; speaker loss meansdisposed between said speaker means and the input of the associated oneof said remote stations for selectively inserting a second predeterminedamount of loss into said audio loop; voice detect means for generating avoice detect signal when the input level to said microphone meansexceeds a predetermined threshold; and loop control means for activatingsaid microphone loss means in a first mode and activating said speakerloss means in a second mode, said loop control means operating in saidfirst mode in the absence of said detect signal and in said second modewhen said voice detect signal is present, said control means inhibitedfrom operating in either said first or second mode when the desiredoutput level of said level control means is less than a predeterminedlevel threshold; said first predetermined amount of loss in saidmicrophone loss means being a function of the setting of said levelcontrol means such that the loss inserted increases as the desired levelset by said level control means increases above said predetermined levelthreshold.
 16. The conferencing network of claim 15 wherein theincreased signal level to said microphone means that activates saidvoice detect means is derived from an external voice signal summed withthe output of said speaker means and said microphone means.
 17. Theconferencing network of claim 15 and further comprising:handset meansfor increasing the audio path loss through said transmission mediumbetween said microphone means and said speaker means; and means forinhibiting said loop control means from activating said microphone lossmeans or said speaker loss means when the audio path loss between saidmicrophone means and said speaker means is increased.
 18. Theconferencing network of claim 15 and further comprising means forvarying said first and second predetermined amounts of loss in saidmicrophone loss means and said speaker loss means as a function of thenumber of remote stations in the conference such that the first andsecond predetermined losses are increased for each added one of saidremote stations.
 19. A method for communicating between a plurality ofremote stations, comprising:providing a full duplex audio path from eachof the remote stations to a central switching network with a first pathfor transmitting audio from the associated remote station to theswitching network and a second path for transmitting audio from theswitching network to the remote station, the first and second pathisolated from each other; interconnecting select ones of the remotestations in a conference at the switching network; summing the audio atthe switching network received from all of the remote stations fortransmission to a select one of the remote stations such that eachremote station receives the summed audio from all of the other remotestations without having its own audio summed therewith; receiving audioat each of the remote stations from an associated transmission mediumand transmitting it to the associated first path of the full duplexaudio path; receiving the audio at each of the remote stations from thesecond path and outputting the received audio to the associatedtransmission medium; the first and second paths in the full duplex audiopath associated with each of the remote stations and the audio paththrough the associated transmission medium forming an audio loop whentwo or more remote stations are connected in a conference; andselectively increasing the loss in the audio loop at the associated oneof the remote stations to prevent the audio loop from oscillating due tovariations in the loss in the associated one of the transmission mediumsin the level of the audio received from external sources in theassociated transmission medium, the loss increase being determined bythe number of remote stations in the conference.
 20. A method forcommunicating between a plurality of remote stations,comprising:providing a full duplex audio path from each of the remotestations to a central switching network with a first path fortransmitting audio from the associated remote station to the switchingnetwork and a second path for transmitting audio from the switchingnetwork to the remote station, the first and second path isolated fromeach other; interconnecting select one of the remote stations in aconference at the switching network; summing the audio at the switchingnetwork received from all of the remote stations for transmission to aselect one of the remote stations such that each remote station receivesthe summed audio from all of the other remote stations without havingits own audio summed therewith; receiving audio at each of the remotestations from an associated transmission medium and transmitting it tothe associated first path of the full duplex audio path; receiving theaudio at each of the remote stations from the second path and outputtingthe received audio to the associated transmission medium; the first andsecond paths in the full duplex audio path associated with each of theremote stations and the audio path through the associated transmissionmedium forming an audio loop when two or more remote stations areconnected in a conference; and selectively increasing the loss in theaudio loop at the associated one of the remote stations to prevent theaudio loop from oscillating due to variations in the loss in theassociated one of the transmission mediums in the level of the audioreceived from external sources in the associated transmission medium,the step of increasing the loss in the audio loop comprising:selecting adesired level for output from the received audio of the second path ateach of the remote stations, the level controllable at each of theremote stations; generating a voice detect signal when the audio levelin the transmission medium exceeds a predetermined threshold;selectively increasing the loss between the transmission medium and theaudio transmitted on the first path in a first mode such that firstpredetermined amount of loss can be selectively inserted into the audioloop; selectively increasing the loss between the audio received fromthe second path and the transmission medium at the associated one of theremote stations for selectively inserting a second predetermined amountof loss into the audio loop; and controlling the opertion of the remotestation in the first or second mode such that the remote stationoperates in the first mode in the absence of the voice detect signal andin the second mode when the voice detect signal is present, operation inthe first and second mode inhibited when the selected output level isless than a predetermined level threshold; the first predeterminedamount of loss being a function of the setting of the desired level suchthat the loss inserted increases as the desired level increases abovethe predetermined level threshold.
 21. The method of claim 20 whereinthe increased signal level in the associated transmission medium isderived from an external voice signal summed with the received outputlevel from the second path.
 22. The method of claim 20 furthercomprising varying the first and second predetermined amounts of loss asa function of the number of remote stations in the conference such thatthe first and second predetermined losses are increased for each addedone of the remote stations.